To be fair, the FRITZ suite also wants to (and does) support Cable internet (afaik the only non-ISP-supplied modem or router-modem you could even buy in europe)

To be fair, the FRITZ suite also wants to (and does) support Cable internet (afaik the only non-ISP-supplied modem or router-modem you could even buy in europe), DECT, and a range of 433MHz home automation products. And of course, you mentioned their homebrew mesh stuff. So there's a lot of non-standard tech available in those boxes and it is no huge surprise that this is kept proprietary.