I can imagine that they are on Facebook simply because the audience is very big compared to gab. They can (unfortunately) reach more people on fb
They're on fb because fb needs a complete social graph and hate drives engagement.
If that's the case then, and if nothing changes then Facebook would now be a safe haven for these nazis and I don't see Facebook doing anything to get rid of them.
The other day I still saw nazi accounts on Twitter, so as far as I can see, nothing is being done about that either.
Nazi accounts are everywhere but the report shows that on FB they are also selling things and FB shoul take serious action against those accounts.