The simple answer is that it doesn't help with tendonitis. Randonmized controlled trial studies show that there are no benefits with kinesiotaping and tendonitis.
I know you do not ask for medical advice and you said that you trust your physiotherapist, but I want to ask you this question, "Why do you come up here and questioning your physiotherapist, and not asking your physiotherapist about this treatment plan right in front of him/her?"
The truth is that you need to know what causes the "mild posterior tib tendonitis - PTT in the first place?"
This can be from overtraining, bad shoe wear, poor foot alignment, tightness in the Achilles tendon and hamstrings, etc.
With this being said, the best treatment for any tendonitis is eccentric strengthening, proper training regimen to include rest and hydration, deep tissue massage, ice massage for pain management, and of course lots of flexibility.
Ask your physiotherapist again on how did you get this problem in the first place. Then go from there. Remember if the tire is low, and you keep filing it up with air, but never wonder why it was low in the beginning, then you only fixing the symptoms, not the problem. You might have to do this for a very long time if you keep thinking this way.
Hope this helps!