In essence, that is exactly what it is: an RBF transaction is simply a new, distinct transaction.

The RBF aspect, the rules associated with it, matter however. Full nodes maintain a list of transactions they expect to confirm (called their “mempool”), and in order to enter the mempool, and be relayed to other nodes, there are applicable rules.

One of those rules is that normally, an existing transaction in the mempool cannot be replaced by another. When RBF is enabled on the original, this is allowed under certain circumstances.

In short, while the replacement transaction is valid regardless of whether the original opted in the RBF, that does not mean it will necessarily be accepted by nodes and propagated across the network to miners.