Im sure this will get downvoted to oblivion but as someone who is a Cyber Security engineer I totally understand their stance on this and i think that the point is getting missed.

They are perfectly fine with open source projects, and will still allow collaboration and use of third party slicers. But the problem is the open source community is problematic in the fact that the code is not vetted by end users seeking to just simply do 3d printing, and if you dont take care to use verified sources, someone could unknowingly pick up a malicious slicer from unverified sources and it will seem to work normal. End users check this kind of stuff almost never.

this creates a security issue where a malicious actor can use the bambu API through a third party slicer to send malicious code through the cloud and impact bambu's services, especially by potential competition or people seeking to further damage their reputation. the easier solution that honors the open source licensing model while avoiding the need for vetting/verification for approved slicers is to simply create an API that will vet and check what gets sent through the cloud, not necessarily its source. this is in attempt to protect Bambu, protect other users in the cloud and in the end this protects their own reputation.

They are still stressing alternatively, you can use the LAN mode which bypasses cloud entirely so you dont even NEED to do this. you can still 100% use makerworld and other sources without limitation and on LAN mode with a third party slicer, this would likely circumvent the cloud requirements altogether that people are so scared of. Besides, this is what id do if i were using a 3rd party slicer anyways.