Semantically, the difference in their singular use (prior to very recent evolutions) is that they was rarely used for specific known humams, and it was never uses for humans (but frequently for living things with sex and to whom grammatical gender corresponding to sex could also be used, in cases where the sex was unknown or unimportant, similar to the use of “they” for humans.)

So, while I find the idea of “they” as gender neutral but “it” as emphatically ungendered conceptuallt interesting, I don't really think it reflects historical usage differences.

OTOH, I suppose there could be debate about different presumptions of social gender vs grammatical gender here, too.