So, the arithmetic and geometric mean agree when the inputs are equal, and, the more unequal they are, the lower the geometric mean is.
I note that the subtests have ceilings, which puts a limit on how much any one can skew the result. Like, if you have 10 subtests, and the max score is something like 150, then presumably each test has a max score of 15 points. If we imagine someone gets five 7s and five 13s (a moderately unbalanced set of abilities), then the geometric mean is 9.54, while the arithmetic mean is 10. So, even if someone were confused about whether the IQ test was using a geometric or an arithmetic mean, does it make a large difference in practice?
The people you’re arguing against, is it actually a crux for them? Do they think IQ tests are totally invalid because they’re using an arithmetic mean, but actually they should realize it’s more like a geometric mean and then they’d agree IQ tests are great?
One argument I’ve encountered is that sentient creatures are precisely those creatures that we can form cooperative agreements with. (Counter-argument: one might think that e.g. the relationship with a pet is also a cooperative one [perhaps more obviously if you train them to do something important, and you feed them], while also thinking that pets aren’t sentient.)
Another is that some people’s approach to the Prisoner’s Dilemma is to decide “Anyone who’s sufficiently similar to me can be expected to make the same choice as me, and it’s best for all of us if we cooperate, so I’ll cooperate when encountering them”; and some of them may figure that sentience alone is sufficient similarity.