I also ran into this issue a while ago when I was trying to implement a custom matter bridge. I reported this to Homey and their reply was basically “we are not sure yet if we are going to allow this”. I would suggest to open a support ticket yourself as well just so they see that there is demand for this feature.
EDIT: Full response I got back in April:
It is indeed not possible to add uncertified Matter devices. This is because the devices are not tested and we cannot be sure that this won’t have consequences.
Perhaps this feature will be available in the future, but I can’t make promises.
I hope this answers your question to your satisfaction, which is why I’m closing your ticket. If you need additional support, please feel free to re-open your ticket by answering this email.
I wish you a great day and a lot of fun with Homey!
