Whatever "racist-adjacent" means, clearly it must mean "not actually racist" because if it was racist, I'm sure you would call that.
I'm doubtful you will convince anyone with that argument: perhaps not even yourself.
Now, if you think the statement "the SPLC is a hate group" is false that's fine. I think there is a debate that is worth having here on HN - especially when tech companies are using the SPLC's hate list to justify their deplatforming decisions. In my opinion, the SPLC's use of defamation and fear-mongering for profit make the statement perfectly justified.
Calling it "racist" or "racist-adjacent" is a rhetorical attack that does not serve the purpose of mutual enlightenment. "Racist-adjacent" strikes me as particularly insidious since implicit in that label is the admission that the thing being labeled is not actually racist.