Let's note that (I would say) 99% of a gm's tournament prep is opening prep. They don't play again a bot for practice - that's not how they prep.
As said, if they can find an opening novelty, it's gold. It doesn't have to be a move that refutes the opening line either, it can just be a surprising (but sound) move that the opponent didn't prep for. The opponent will then have to spend lots of clock time OTB to get to grips with it, and that advantage can make all the difference.
This is what GM's battle to do all the time.
Also, let's note that super GM's have a team of seconds.
They would be crazy not to use Leela as just one of their tools (and they'd have many tools).
They would go even further and make sure they were familiar with novel lines Leela might come up with against their opening prep, in case their opponent used Leela for their opening prep.
I'd be astounded if Leela isn't being used all over the place at top level chess (even as she is right now).