p. 166
The core book doesn't seem to differentiate between copying an ability and an art summa. Did I miss sth?

footnote: ArM5 mentions a Parma summa on p. 166! I know many of us believe they don't exist...

As I read it, the only difference between Summae in Arts and Abilities is that mere mundanes can't copy Art Summae.

BTW the page you make a reference to is not regarding Summae but texts in general so also Tractati. But I follow your reason that Ars books published since the core book more than hint that Parma Magica is so great a secret that having books about it would be a bad idea. One of my sagas features a Parma Magica Summa, but it is well known for being an exception to the rule, and demands strict security measures be taken, and the Tribunal as a whole will deal harshly with those flaunting this. We had a small plotline with an Arab Ex Miscellanea member of their covenant make off with this to deliver it to their enemies...

As long as they have any score in Magic Theory IIRC?

Whereas writing Ability summae is 5 times the level, yes. I don't know if it is voluntary or not.