The unit measure, however, may be entirely defined by the scale in question. Gleb could say "my unit measure is 1/9 of the full range of possible answers on my Meaning Measuring Scale". That would be pretty useless, but so is "my unit measure is 1/15 of one standard deviation on my Cleverness Measuring Scale".

Perhaps it's better to reference scores to standard deviation rather than the full possible range -- I guess it depends on the particular case. The other advantage IQ has is that there are now lots of IQ tests and they tend to get somewhat correlated results (suggesting that maybe they're measuring something real) that also correlate with other interesting things (suggesting that maybe the real thing they're measuring is useful) and they get used quite a lot (so that if you quote an IQ score there's a good chance that the people you're addressing will understand roughly what you mean).

Those are all genuine (or at least possibly-genuine) ways in which IQ scores are more useful than Gleb Meaningfulness Metric scores. But I don't see that IQ is any better off than Gleb Meaningfulness Metric in terms of unit-measure-having. One IQ point doesn't correspond to a fixed increment in thinking speed or memory capacity or ability to solve any particular kind of problem, or anything like that; it's just a certain fraction of how much variation there is in one kind of brainpower score. One Gleb Meaningfulness Metric point, likewise, is just a certain fraction of how much variation is possible in one kind of feeling-like-your-life-has-meaning score.