"Going back to 1.5 actually means less damage than not going back."

That's a different claim, but also no. Let's imagine we go 1.5 -> 2 -> 1.5 in a few decades (and keep in mind capture is more difficult, so the 2nd transition will take longer), compared to just 1.5 -> 2. In the return case, we stress all the ecosystems twice by change, rather than once. So it's not at all clear it's less damaging.