This seems hard to prove a direct causal link.

Remote employees are also in demand enough to negotiate / find a role with remote work. They probably then are experienced, senior, and better compensated than their peers.

It could be that the underlying cause is being more in-demand in a position to negotiate remote work, not remote work by itself.

Where am I wrong?

Many companies that are offering remote work knowingly provide lower salaries. And many people agree because added flexibility has a price too.

This is a survey of 20,000 health care workers in one organization. I doubt that that many of them are senior.

Proving a causal link between being remote not having to commute is easy.