Who said they didn't? Not sure what "very little" attribution means, but presumably it doesn't mean "none."

I think I misunderstood GPL per a comment above so it's all kinda moot anyway, but my original understanding was that it was a copyleft license, so wishing it was GPL would be equivelant to wishing Anthropic had not used that work at all.

Attribution is certainly polite, and I sorta doubt Anthropic of all groups is intentionally witholding it for h-score or prestige, but that's all a much lower-stakes conversation than potential license violations. AFAIU.