Good question Xue.
You are correct when you say that the tendon is made of collagen and not synovium so it should be immune to attack by RA.
However the tendon has a synovial sheath (well at least most do) and pin this case it is tibialis posterior and that certainly has a synovial sheath. The synovium in the sheath is attacked by the RA and a tenosynovitis results. The inflamed sheath is a hostile environment for the tendon and we see tendinosis and eventual rupture ensue.
This is a common problem in RA patients.
Your images are beautiful and a classic example of RA disease leading to tenosynovitis and eventual tendon failure.
You will also see other synovium containing structures coming under attack such as bursae.
Nice one Xue,
Steve