Well, they at least should be able to explain why on some level beyond "I'm used to it the other way". I can explain the problems I see with A/D; some people might not care, but they're at least something beyond pure taste issues. As an example, though I might argue whether its as severe as some say, when discussing roll high versus roll low systems, the argument that subtraction is harder for people is sound; its a known fact. Then you can get into the question of whether its so much so, so frequently, to counterweight other advantages.
When we're talking about a leisure activity, if someone says that they prefer the 1e matrices to ascending AC
as a subjective preference, I see no basis for demanding an explanation, or claiming that the preference is invalid without it. That would feel like expecting someone to provide an objective justification for having a favourite colour. The whole point of subjective preferences is that they've very subjective, and not always easy to explain logically. Why does X taste better than Y? Sometimes, I might be able to point out something specific but, sometimes, I don't know, it just does.
The bottom line for me is that, even if there are instances where technically, we might be able to say that one very specific mechanic is objectively better than another, 99% of the time, when someone is claiming a particular system, mechanic, game style, etc is objectively better/best, they're really just trying convince people (or themselves) that their subjective preferences are actually objective truths. The fact that most people find addition a little easier than subtraction often doesn't tell us much of anything that's useful as, in many instances, this fact is going to be overshadowed by dozens upon dozens of other much more relevant (and subjective) facts about the system, what it's setting out to do and how it's explained. And then, of course, one needs to account for the people for whom there is no meaningful difference in their ability to add or subtract and probably even some extreme outliers who find subtraction easier. Their opinions don't cease to count just because they're a minority.
The one main exception is when you are assessing something according to clear, very specific criteria. If my aim is to minimise any addition or subtraction, then it's probably fair to say that D&D 5e-style adv/dis is objectively better than a host of specific, variable modifiers. However, whether it's better
overall depends entirely on how important it is to you to minimise addition and subtraction and whether the things lost in doing so matter to you.