They mean you don't know and can't reason about lock state. To design concurrent systems with good performance and correctness, you need to be able to reason about lock state statically.
But you can reason about lock state statically with recursive locks.
I have a function A, that accesses and modifies a data structure X. I have another function, B, that modifies X, but that also calls A to do part of its work. And I have a function C that calls A, but never accesses X except through A.
I can perfectly well statically reason about a recursive mutex that both A and B take to protect access to X. It's not magic.
I mean it exists for a reason, there are some rare cases where it might be the right tradeoff to use it. However, these cases are pretty rare: what’s far more likely is that the critical region is not considered well and the recursive mutex is a patch for “I don’t really know what’s going on here, let’s just use the thing that works”. You can form your own opinion on this but I actually find it common for people to avoid designs like yours specifically to avoid the recursive lock; usually the alternative is about as ergonomic and it reduces the need to reason about recursive locking.
