[ 2012, Kona ]

This does not specify a concept; it specifies requirements on the concrete mutex types.

[thread.mutex.requirements.mutex]/6, fourth bullet requires the return type of m.lock() to be void.

This is over-constrained. The true requirement is that the standard library ignores any value that the function returns. Yes, allowing non-void return types means that users can't store a pointer to this member function. No, that's not the least bit important.

[See also the discussion following c++std-lib-31318]