Why doesn't removing (rm) files free up disk space?

No hard links (done with ln), I am certain of that. I know of APFS and its deduplication and I am quite sure that there is no other location for these files on the same volume still in existence.

Besides, deduplication would mean that a tally of all file sizes with du (or OmniDiskSweeper) run as root could only be larger than size used. The opposite is true: ODS tells me I have 255GB in use, df tells me I have 463GB in use: 200GB is missing. See 200GB free space lost from APFS boot drive

Well, one thing I do know: what df tells me is true in the sense that if according to df the file system is full, no more files can be created and the system crashes. Both du (Apple provided tool) and ODS tell me there is far less actually on the system. The difference between the two is not normal, nor is removing 33GB of files and not getting free space, nor is du/ODS reporting more free space than df, as APFS does deduplication.

This was it. I removed the oldest, but that did not have an effect. I removed 10-12 more, and suddenly, the space is freed.

But this is worrying. Because the local snapshots fill up the boot disk and can play a role in bringing the system down (which has happened to me). Can I stop Time Machine to create these local snapshots? Or is there a way to make sure they are not this risky?

Since Apple has dropped support for installing on/upgrading of (and soon booting of) a RAID drive, I have — before installing High Sierra — changed my setup from a dual 500GB SSD in a mirror RAID to a setup where one disk is boot and the other is a time machine of that boot (apart from that I use another backup solution to backup this externally, which I already had when I was using RAID).

Here is the diskutil output (I wonder, btw, why the boot disk is only 465GB as reported by df, where is the other 34GB? And what is VM?):

dumbledore:~ sysbh$ diskutil list
/dev/disk0 (internal, physical):
  #:                      TYPE NAME                    SIZE      IDENTIFIER
  0:      GUID_partition_scheme                        *500.1 GB  disk0
  1:                        EFI EFI                    209.7 MB  disk0s1
  2:                Apple_APFS Container disk2        499.9 GB  disk0s2

/dev/disk1 (internal, physical):
  #:                      TYPE NAME                    SIZE      IDENTIFIER
  0:      GUID_partition_scheme                        *500.1 GB  disk1
  1:                        EFI EFI                    209.7 MB  disk1s1
  2:          Apple_CoreStorage DumbledoreTimeMachine  499.8 GB  disk1s2
  3:                Apple_Boot Boot OS X              134.2 MB  disk1s3

/dev/disk2 (synthesized):
  #:                      TYPE NAME                    SIZE      IDENTIFIER
  0:      APFS Container Scheme -                      +499.9 GB  disk2
                                Physical Store disk0s2
  1:                APFS Volume DumbledoreRoot          262.8 GB  disk2s1
  2:                APFS Volume Preboot                19.6 MB    disk2s2
  3:                APFS Volume Recovery                516.1 MB  disk2s3
  4:                APFS Volume VM                      24.6 KB    disk2s4

/dev/disk3 (external, physical):
  #:                      TYPE NAME                    SIZE      IDENTIFIER
  0:      GUID_partition_scheme                        *2.0 TB    disk3
  1:                        EFI EFI                    209.7 MB  disk3s1
  2:                  Apple_HFS RNAStore1              2.0 TB    disk3s2

/dev/disk4 (internal, virtual):
  #:                      TYPE NAME                    SIZE      IDENTIFIER
  0:                  Apple_HFS DumbledoreTimeMachine  +499.4 GB  disk4
                                Logical Volume on disk1s2
                                48EFF1CC-91F9-493D-BE61-2284194BDC93
                                Unlocked Encrypted

The Time Machine disk contains my Time Machine backup of my boot disk. But the local snapshots are created on the actual disk that is being backed up using APFS block deduplication. These do not take up room, unless the file system changes a lot within 24 hours (I think it keeps one every hour). The problem seems to me that the local snapshots are superfluous when Time Machine is running continuously (as it does, because the target is an internal disk).

Sadly, it cannot be turned off because it is the non-removable part of how TM works now: it first creates a local snapshot before moving that to another volume.

It would be nice if TM had an option to clean up local snapshots as soon as they have been used to create a remote one.

I should have said: I already ran disk first aid (ran the system in TDM and mounted in on another Mac and ran first aid) and it told me the disk (the file system) was OK.

I would prefer to know how to turn off local snapshots as they bring me very little compared by the actual TimeMachine backups on the SSD next to it in the same machine.