I'm gonna guess they meant the other direction.

Acquiring a massive library of unlicensed IP, the best-known such library in the world, would also seem to destroy arXiv's credibility with those in power and I expect would be the end of arXiv.

sci-hub is still used by professionals even though it is illegal

Professionals in wealthy countries have access to licensed, legal sources. Do you mean people in other countries?

When I had access to academic journals through my (western, wealthy) institution I would still mostly use scihub.

The simple reason was that I got conditioned to doing that because the login wall to doing it legitimately was often too painful. At a certain point my workflow switched to checking scihub first, then going the legal path if scihub failed.

It saves a lot of time, and I suspect it is a lot more common than most people think.

sci-hub's legitimacy.