Ah!

I took “come to racism” to refer to people who used to be non-racist[1], but become racist[1] as adults.

Yeah, with that unpacking, I find the claim much more plausible.

many (most?) randomly-chosen racists[1] probably have been so ever since they've had any opinion either way on the matter, which they probably uncritically absorbed from their sociocultural environment while growing up and have had it cached ever since.

Yeah, that's my expectation.

These two groups of racists[1] are probably very different

No doubt.

most “converts to racism” are the way Eugine_Nier says.

I find that much more plausible than the claim that most racists[1] are the way Eugine_Nier says.

I'm not sure I believe it even so (as compared to, say, converting to racism after a traumatic experience with a member of race X), but at this point I'm just telling just-so stories about hypothetical people I don't have much experience with, so I don't put much weight in my own intuitions.