As far as I can tell, they're just objecting to use of their cloud service. You can fork their software and use it with your own printer just fine, they just don't want you to use it with their cloud service, which its own terms of service for access.
I think it's an odd hill for them to die on, but it's not a totally unreasonable position - the cloud is other people's computers, other people can have rules about what you can do with their computers. Just because a client is open-source, doesn't mean you're allowed to use the server.
If you're using developer mode running everything locally (or remotely over your own VPN, like the author here) then I think this makes zero difference.
I think it's an odd hill for them to die on, but it's not a totally unreasonable position - the cloud is other people's computers, other people can have rules about what you can do with their computers. Just because a client is open-source, doesn't mean you're allowed to use the server.
If you're using developer mode running everything locally (or remotely over your own VPN, like the author here) then I think this makes zero difference.