I've gotten two of these mails and still do not understand what they're asking me.

So I see three options:
1. they're just bad at explaining; or
2. their lawyers told them to keep it vague; or
3. they're doing malicious compliance and/or lobbying by making it intentionally confusing, so authors and publishers blame the EU

(3) would then help them with other aspects of DSA that are unrelated to books.

If I had to bet it's (1) though - the iBooks department is probably too low on their list of things to spend any attention on, so they don't bother making this compliance step easy to understand.