Glossary
APFS snapshot
A point-in-time copy of an APFS volume, used by local Time Machine snapshots and some system updates.
APFS can keep a snapshot: a preserved view of the file system at a moment, cheap to create because unchanged data is shared. Time Machine uses local snapshots. They can make free space look smaller until the system deletes them or you do, with a tool that is explicit about snapshots.
Snapshot deletion is not one of the behaviors described for Disk Housekeeper. Smart Clean is about developer caches, node_modules, sorting, and watched folders, with a preview and the Trash. If snapshots are the space you need back, that is a separate macOS question, not a hidden mode of this app.