Is posterior tibial tendonitis the same as posterior tibial tendon dysfunction?

Yes: And no. In reality they are. However, there are semantics involved. Most of the times the tendons are really degenerating and not inflammed. So tendonitits, is not really always the correct terminology. But! yes all post tib issues can be grouped under tendon dysfunction.

Very close.: Tendonits is a bit more acute and sharply painful. Dysfunction (pttd) refers more to the overuse/overstrecthing of the tendon for mechanical reasons. A person with pttd can also have tendonitis, whereas a patient with tenodonitis may or may not have pttd.