[sparky_z]

It's true that there is an ambiguity in this particular sentence you've constructed. But that's always going to be possibile when pronouns are involved. Just pick two subjects that share a pronoun, put them in the same sentence, and then use the pronoun. Uh oh!

What if "Harper" were the name of a company? Rejecting singular "they" wouldn't save you then. I would propose that whatever remedy you would suggest for the "Harper Inc." case, you go ahead and apply to this one as well.

[ibejoeb]

I think the point is that the author of the piece has also constructed sentences wherein the ambiguity can be argued away. That doesn't make it more useful, though.

Replacing "he" and "she" with "they" both removes information and introduces an ambiguity in number.

downvoter: am I wrong?