This is strange. Usually "studieren" is Diplom阶段的学习. (In general, Diplom=Bachelor+Master). The period after getting Diplom is not "studieren". Especially,
after getting Docter degree (Promotion) it is definitely
not "studieren". It should be "arbeiten" (work).

It is most probably provided by himself.

This in fact could happen. One year ago I received
a letter from HU Berlin to ask me if I have studied
and gotten degree in HU and asked to fill some forms,
though I just visited HU privately for several hours.