I think he disagrees with the claim that “flatMap does not guarantee any serialization”.
Apparently (judging from that stack overflow answer) if you call flatMap(maxPublishers: .max(1)), flatMap only passes .max(1) demand upstream, so it will only get a single “inner” publisher to subscribe to. Then flatMap waits for that inner publisher to finish before passing another .max(1) demand upstream. So flatMap(maxPublishers: .max(1)) is only ever subscribed to one inner publisher at a time. It therefore concatenates the sequences of values from the inner publishers, instead of intermingling them.
This is a really useful insight that is not at all obvious from the documentation. I look forward to the day Combine has complete documentation specifying precisely what all its many operators actually do.