Mutual exclusion is a requirement on the behaviour of multiple parallel processes that use shared memory. The sentence that confused you refers to this: common memory = shared memory.
The mutual exclusion requirement states that at most one process can be in its critical section at any given time (a critical section is a piece of program code that uses a specific resource residing in shared memory). The requirement does not say anything about how it can be achieved, only what has to be achieved.
Semaphores and monitors are devices with which mutual exclusion can be enforced, so they answer the "how" question. Both have to be used according to their own rules, but both can enforce mutual exclusion.
Hope this helps.