In that case, what difference does the underlying block storage format really matter? If rsync didn’t work, general applications could easily be expected to have issues too.
It's the metadata that's really important. Just copying a file, while faithfully preserving metadata, is actually a feat on HFS+. That is why popular tools such as BackupBouncer (to test the integrity of backups) came to be.
I think you missed my point, everything rsync does is via the very same system calls any other app could use to originally get or set that metadata. As such, the kernel file system layer abstracts the block storage away from you. This is very different than say Shirt Pocket’s approach (linked to in the article) where they are directly accessing/modifying/creating low level block devices. I’d expect what they are doing to be SIGNIFICANTLY harder. The reason why OP is generally positive is because essentially the only thing they had to do to support APFS is a bunch of testing, unless there’s a part to this story they didn’t tell.
I honestly have no idea what you are talking about. Shirt Pocket would not "access/modify/create low level block devices". They would also be working at the file layer. If you mean that you think that they are implementing their own way of reading the filesystem through the POSIX node, I really dont think that they are, and also it would be crazy to try to do that.