Increasing Priority Inside Mutexes

I am new to pthreads, and I am trying to understand the output of a C language code, could you please help me with that?

In the program, basically, thread A is executing after being locked, with the command: pthread_mutex_lock(&lock);

Inside this mutex the priority of a second thread (Thread B, which so far had the same priority level of Thread A) is then increased.

How can the program start to execute Thread B if, supposedly it was locked inside the mutex?

Increasing Priority Inside Mutexes