This button is not displayed unless the updates are already downloaded (“prepared”) and ready to be installed. If it doesn’t immediately begin updating, that’s a bug.
It looks like that for atomic desktops and the legacy RPM version it is slightly different, the update process, and the wording not fits perfectly on both versions.
Atomic downloads the new read only file and a restart is already the update.
While on Legacy RPM the update has to be locally stored and be rebooted to initiate the update ?!
Maybe just labeling the button as “Restart/Update” would express this better?