Good catch, that does indeed look like a mistake in the spec. Everything past the first sentence of that error description is suspect, honestly (seems like it was naively adapted from the example in [1] but that example isn't relevant to the missing-semicolon-after-character-reference error).

Will submit an issue/PR to correct it when I get a chance.

[1] https://html.spec.whatwg.org/multipage/parsing.html#named-ch...