[petewailes]

Obviously Ethan knows this and just isn't going into it because this is more a history lesson than a theory lesson, but the same applies to white keys. So B♯ and F♭ are perfectly valid notes. C♯ major for example contains B♯, despite that there's no black key between B and C.

[chrismorgan]

For that matter, F𝄪 (F double sharp) and A𝄫 (A double flat) are both legitimate alternatives/equivalents to G in some situations, by extrapolating the sequences. (And if you extrapolate far enough, any note has multiple possible alternatives—for example, you could get a G♮ that’s kinda more G♯♭ or G♭♯ than just straight G, to use super fuzzy terminology.)