Where in that book is the rule stated? I ask because I have heard the author explicitly state that multiple assertions are fine (using essentially the same explanation as
TrianguloY did in this comment: https://news.ycombinator.com/item?id=33480120).
Chapter 9 talks about unit tests and there is a paragraph called 'Single Assert per Test', where he says it is a good concept but that he is not afraid to put more asserts in his tests.
That paragraph is followed by 'Single Concept per Test' where he starts with: 'Perhaps a better rule is that we want to test a single concept per test.'
That maps better with the lectures I've seen of him on YouTube, and I concur with it.
When I first wrote tests years ago, I would try to test everything in one test function. I think juniors have a tendency to do that in functions overall - it's par to see 30-100+ line functions that might be doing just a little too much on their own, and test functions are no different.
It feels as if folks are splitting hairs where a haircut (or at least a half-hearted attempt at grooming) is required. Use good judgment, and do not blindly follow "rules" without understanding their effects.
