The word "tree" already has a very general and overloaded meaning within the context of data structures. Presumably, this "rope" implementation is using a tree structure internally, but that's true of many thousands of things; it's a basic foundation of data structures and computer science. I assume "rope" was chosen as a tongue in cheek alternative to "string".

'Ropes' are built using 'trees'. They're more specific than trees.

Otherwise, why do we say 'trees' when we could use the more general term 'graphs' if that's your argument?

I always figured rope was just meant to imply that it was bigger than string.