In iMessage and Signal contacts are identified by the phone number.
Delta Chat cannot match the contacts in the address book to the same as the contact in Delta Chat because there are no phone numbers in Delta Chat. Maybe it is possible to match by the email address, but for encrypted chats contacts are primarily identified by their cryptographic identity rather than email address and we want to make it possible to migrate to other email addresses or use multiple email addresses with the same profile in the future.
How does it work in Signal for contacts that were added by “username” or via shared groups and for which you don’t have the contact phone number? Can you select such contacts in iOS settings?
I imagine a good and simple workaround could be to (optionally) allow users to save a contacts phone number for a/each Delta Chat contact.
Then Delta Chat can make the “link” and properly handle Focus Mode for those contacts.
After all, this is most important for a few “exceptions”. You don’t need to do this for all users.