At least in one version of the documentation patch, there's mention of a watchdog timeout that unloads the user scheduler if it fails to make forward progress for some period of time.
Correct -- the purpose of the watchdog is to account for buggy schedulers that fail to schedule runnable tasks. scx_rustland has special logic to track the user-space scheduling task, though. If it incorrectly failed to schedule it when it had no more runnable tasks to run, then the watchdog would eventually kick in, boot out scx_rustland, and revert back to EEVDF.