I dunno exactly what fork is doing. If it just makes a new note as a copy of an existing note, with a link in the new note, that is fine. If it is using kind 1010 event versioning, that won't be recognized by other clients meaning it creates broken experiences.
Discussion
There is a subtle difference :
In #Amethyst , when I fork a note , it is presented to me as a copy , ready for the edit .. which is convenience ..but the crucial thing is there is a (system generated) link to the original note , which as a user I am abstained from edititing because the link shows up after I already posted .. I need to check if the link is visible in #gossip ..
In a way , I am forced to link new note to to the original . I guess that is the subtle difference between copy/paste and fork !