I would guess they simply applied their existing word game dictionary — probably the same one they use for “Spelling Bee”. It doesn’t include slurs, uses US-sp

I would guess they simply applied their existing word game dictionary — probably the same one they use for “Spelling Bee”. It doesn’t include slurs, uses US-spellings (at least in the US), and intentionally doesn’t include “obscure” words, though obviously that’s a judgement call. (Spelling Bee has an email link if you want to dispute an exclusion — I believe I helped get “ichor” added to the list.)