I believe it's a soft fork where they're treating Iced as upstream. Having their own fork just means they can land changes quicker.

The last iced release was September 2024, more than a year ago. I imagine they had constraints that made it impossible to wait for upstream patches to be merged (if at all) and it was simply more tenable to fork and worry about merging later.

They have needs that are specific to COSMIC, so they maintain a separate fork, but they update their fork to match the latest upstream release.

They also contribute to iced indirectly via cosmic-text and other crates.