On Day 1 of the Withdrawal Agreement, the UK is legally out of the EU. A few years later, after the transition, or after the Backstop is no longer necessary, we're out in literal terms. The period in between Day 1 and Day 2376 or whenever is, to be sure, rotten for the UK. But signing the Withdrawal Agreement (WA) does set us on a path towards leaving. A long, winding, frankly humiliating path, but a path to leaving all the same.
Maybe it is a semantic thing. But still, the thing that Leave-voting critics of the WA can't ignore is that, whether it is two, five, or ten years after it comes into effect, the UK ends up out of the EU. It is, therefore, far from a "Remainer's Brexit".