How do I delete or show the size of a non-TimeMachine APFS snapshot
I received a reply from the author of ChronoSync, and he helpfully explained the following option to manage snapshots.

To list snapshots:

diskutil apfs listSnapshots /


To delete snapshots:

diskutil apfs deleteSnapshot / -name &lt;name_of_snapshot&gt;


This seems the better option, which unfortunately doesn't seem listed elsewhere (all other guides I found only mention tmutil).
I couldn't test, since I already deleted it with CCC.