Yeah no, that's not how it works. On signing the CLA and contributing code to Proxmox projects you agree that you hold the rights on that code to do so (i.e., implemented it yourself, or somebody else gave you the rights on that), so just taking some code and submitting it on their behalf doesn't "magically" removes copyright.And what if someone else does it, so not proxmox or the poster of arbitrary code themselves?
I mean, in practice this is naturally more muddy and a lot of changes are probably to small to warrant a clean room implementation, but we're pretty strict to avoid muddying our code base, and it's strong AGPLv3 license - which helps to keep Proxmox projects alive and well funded in the first place.
I think, if the original authors really aren't interested in up streaming this (or don't have the time for it, all valid reasons after all) it would be probably quicker to just reimplementing this without looking at their patches - for that it might be helpful to open a Bugzilla enhancement request for qemu-server and add some infos about what the goal is and what cloud-init properties are required (ideally with some reference to cloud-init docs or code resources).