Looks like there is no documentation around the two. Yes, under the hood both are flexvol and snapdiff by default supports flexvol. However, the question is - Does Snaplock (WORM Filesystem attribute) allows for changes to the volume/snapshots via 3rd party Backup application using exposed snapdiff API? This is b'cos the way SnapDIff differencing engine works depends upon how it is integrated with 3rd party Backup software, what APIs are exposed, but it general it does the standard stuff such as:
Backup Software using SnapDIff API to: 1) Create a base-snap - 1st full backup 2) Then create a diff-snap - Differential/Incremental 3) Next run- delete the 1st base-snap and use the last diff-snap as 'base' and create a new diff-snaps etc. At any time it will keep only most recent 2 snapshot set.
Even though you can tell certain backup software to use 'already present' snapshots for differencing purpose on read-only file-system but this is something CommVault support will be able to answer. Is the SVM under which the Snaplock vol exits is enabled for snapdiff ? I suggest try to configure/backup the volume and see how far it goes, it will give some clues.