How to use tmutil delete on Big Sur, APFS

The Big Sur syntax changed, but you need to specify the volume and the timestamp with -d and -t:

sudo tmutil delete -d /Volumes/Time\ Machine -t 2020-11-18-100936

You don't need to specify the Backups.backupdb folder or the backup machine. I had done tmutil setdestination /Volumes/Time\ Machine previously (where Time Machine is the name of your drive.)

This is a pretty tentative answer, but too much for a comment...

Big Sur & Time Machine on APFS will always require the entire volume. You can't do anything other - see APFS changes in Big Sur: how Time Machine backs up to APFS, and more

APFS doesn’t support directory hard links, so can’t use the same mechanism when storing Time Machine backups. Instead, what appears to function as a form of virtual file system is created using new features in APFS.
The volume assigned the role of Backup appears to be a regular APFS volume, and is protected from normal access, even by root.

I have to admit I cannot investigate this myself to confirm, I don't have a Big Sur-capable Mac here.