It does not make hard links. It makes copy-on-write clones.
No, because it's not actually a hard link -- if you modify one of the files they'll diverge.
Sounds like jdupes with -B
Cursory googling suggests that it's using the same filesystem feature, yeah.
Replacing duplicates with hard links would be extremely dangerous. Software which expects to be able to modify file A without modifying previously-identical file B would break.