The whitelist (I assume you mean the tooltip that’s displayed) is not necessarily up to date. I think that entry has not been changed in quite some time (since 2018 in fact).
So, if the btrfs version on the system supports the flushoncommit mounting option (or other more recent ones) and you want to use it, then you can enter it there. This way, the option is reflected in the Rockstor db (used for managing pools) and visible in the WebUI. Personally, I have not used this option and not seen any issues since it became available, but that doesn’t mean anything.
As you have probably noticed prior to your actions, Rockstor does not manage btrfs via /etc/fstab for various reasons. Take a look at this wiki entry, while it is a few years old, the underlying approach to how Rockstor manages devices has not changed: