RBF is a code fork, and it's a particularly significant code fork because it changes the functionality of bitcoin and requires significant changes to wallet software and other application software that is built on top of the blockchain.

In one sense, RBF poses a potentially greater risk than a blockchain fork, because the damage it can do is local (between cheating Alice's and duped Bob's) and hence likely to be insidious. Blockchain forks are obvious because of their global impact and  have, historically, mobilized developers and node operators to recover from the resulting  system outages fairly quickly so that ordinary users remain unaffected.