I would argue the same. There simply does not appear to be a way to bridge the famous is-ought gap. Claims such as the one that morality is ultimately about the well-being of sentient creatures are really just begging the question. Why should (mostly) human welfare be the yardstick of moral decisions? Ultimately only because we humans say so. And really, that is okay - who else except humans should decide what human morality has to look like? But let us not pretend that such a decision is universally and rationally justifiable.
And let's not get started on other proposed solutions such as divine command theory, which collapsed under the weight of its intellectual incoherence more than two thousand years ago. Even if there were a cosmic tyrant, why should it be moral to follow their arbitrary orders?
No, a more interesting set of questions than whether we can rationally deduce a single set of objectively and universally true norms would be where our actual norms in fact do come from, and how much of them is contingent versus a necessary development. As we will see, the second of these questions is somewhat related to that of arbitrary versus objective morals, only without the claim of a rational derivation.
I am convinced that where our norms and morality do come from is, regardless of what some religious believers would like to believe, mostly biological and cultural evolution. Morality is about dealing with each other, so for them to exist in the first place, sentient beings have to live and interact in groups. It then immediately appears plausible that certain moral intuitions will be more advantageous to the individual than others. A group will share certain responsibilities (e.g. standing guard while others are feeding), and it would be good if an individual could keep tabs on how much of the responsibility it has shouldered compared with the others, so that it is not exploited. That will necessarily favour the evolution of a rudimentary sense of fairness: the most stable solution is that everybody should take their turn, and nobody should do it for too long or, conversely, bow out entirely.
If resources are shared between individuals, for example because each of them has more or less luck with gathering food every day, a very good strategy for each individual to adopt is to extend some trust to everybody and share food at first, but to stop sharing with somebody once it becomes clear that they are trying to cheat. Because this strategy is so simple and efficient, it can easily be imagined that moral intuitions favourable to its development would be selected for. Thus we may easily add "cheating is bad" to our rudimentary sense of fairness, even before we start to build a real society. And so on.
Once a sentient species proceeds to build complex societies with transmission of knowledge from one generation to another, we get another important process that may select between different systems of morals: cultural evolution. Some norms are very obviously more conductive than others to the cohesion and functioning of a society, and to its survival when competing with other societies.
Let us just take certain behaviours and contemplate what it would mean for a society to regard them as either virtues or vices. For example, lying. A society that considers lying to be a virtue and honesty to be a vice will have a hard time communicating, let alone making well informed decisions, and will thus be extremely dysfunctional. It is thus highly plausible that all societies will independently converge on the opposite norms, i.e. lying is a vice and honesty is a virtue. Individuals will still try to get away with lying and cheating, of course, but that behaviour will not be promoted by society as a whole.
The same goes, for similarly obvious reasons, for a lot of other behaviours. No society will flourish if it adopts a system of morals celebrating stealing, cowardice, laziness, or murder of other members of the society. Similarly, if a society knows no form of aid for the poor at all, it is likely to be unstable, prone to frequent disruptions from crime, revolts, protests, and other measures taken by the desperate.
All this suggests that at a minimum the following norms will independently be converged on by all complex societies, whether any of these norms is "objectively and universally justified" in the sense of deductive reasoning or not:
- Distributing chores equally is god, free-riding is bad.
- Cheating is bad.
- Lying is bad, honesty is good.
- Stealing is bad.
- Cowardice is bad, bravery is good.
- Laziness is bad, diligence is good.
- Murder of in-group members is bad.
- Charity is good.
Mind you, that does not mean there would not be a lot of variety. The norms that are likely to be must-haves are only a small part of the norms that a culture can randomly have or not have, as some contemplation of human societies across the globe and across the ages easily demonstrates. There might well be aliens that regularly practice cannibalism or the ritual sacrifice of captives; have very different ideas about private and public property; or have very different taboos, be it that they have none against copulating in public or that they do have one against looking in the same direction as your father in law when it is a certain day of the year.
It gets even more interesting once we consider what a very different biology might mean for the societal structure of an alien species, for example if the females are considerably stronger and larger than the males, if sex determination is density dependent or the sex of an individual changes depending on age, if the species is hermaphroditic, if gravity on its home planet is much higher or lower than on Earth, or if the members of the species have a much harder time killing each other before the invention of projectile weapons than we humans have (just imagine corvids or parrots trying to swing a club or axe with enough force to do real damage before the intended victim takes flight). Lots of possibilities. It is simply unlikely that they will have morals that are completely alien to us.