That's what I was clarifying with 'Yes, it's in the list of guessable words - however it's not a valid answer word'.

They have two lists of words: one is a list of possible answers, and another is a list of extra (valid) words which can be guessed (in addition to the words in the answer list). Sometimes it might be better to use a non-answer word as a guess: the best case gets worse (since you cannot win immediately) but the average case and worst case both get better.

Ah I understand, thank you for clarifying!