Consider how much of work in btrfs is done to just handle the case of modifying existing files—or reducing file system size.. It is basically the reason it uses b-trees. It's in the name!

For example, when dedupping in block level it needs to know (right?) how many times a block is being used, so it can be collected when it runs out of uses.

ISO9660 can also express dedupped (hardlinked) files with the Rock Ridge extensions. I don't know but I'm wondering it could even do block-level dedupping if the generating program abused the format a bit..

A compression benchmark of both filesystems would be of interest in this regard (lzo, zstd and zlib), both read speed and compression wise

Sure, but it sounds like block deduping is only one of several optimizations that DwarFS is able to take advantage of because it’s a read-only FS.