The very real ambiguity of using singular-vs-plural "they" is something that can be commented on and discussed, and is a linguistic weakness of that pronoun's i

The very real ambiguity of using singular-vs-plural "they" is something that can be commented on and discussed, and is a linguistic weakness of that pronoun's increasing popularity. The english language already has that ambiguity surrounding "you", but since the reader/listener is included in a plural "you" it's easier for them to discern singular/plural. It's not quite the same as the catch in one's brain of encountering something that is unusual but information-complete; there can be truly ambiguous situations where the meaning of a sentence is fully changed depending on whether the author intends a singular or plural "they". Thus its use is a valid criticism one can have of a book, where understanding the author's meaning is kind of the point. Of course, introducing that ambiguity intentionally, or getting people used to the use of singular-they and getting better at distinguishing singular-vs-plural through practice and context can itself be part of the author's point :) It's still probably the best non-gendered singular pronoun we have in english.