The question is, how are they supposed to find out about this trap?

I have searched even the Chinese pages, and I find no government source reporting this limitation for Americans and Canadians. Not even the Wuhan Foreign Affairs Office:

http://www.whfao.gov.cn/html/guide/2...04_45988.shtml (search way down for "24")

Not to say it's certainly not there somewhere, but I can't find it.

Yes, if passengers burrow down in social media, they might get an inkling there's a problem here. But really, that shouldn't be expected or necessary.

One possibility is that Timatic is wrong. If the OP wants to pursure this, why not ask the PSB in Wuhan about this directly. It's an off-chance, but there's nothing to lose by asking. Here's the page for submitting questions, with your name in the first box, a heading in the second, then the question itself. You can try simple English.

http://www.whcrj.gov.cn/ask.asp