The entire Sylve bundle (backend + frontend) is ~55 MB, fully self-contained, and doesn’t mess with the base system in any destructive way. You can drop it in and remove it cleanly.
Proxmox, on the other hand, replaces core parts of the system, including the kernel, and its package ecosystem diverges quite a bit from standard Debian. I’ve tried using it on a desktop before and rolling that back cleanly isn’t exactly straightforward.
At that point it’s more of a tightly coupled platform built on Debian than just “a UI on top,” especially when the underlying system is no longer behaving like Debian in the usual sense.