Yes, the green checkmars means that the code is lint-free (up to a point).

The Jetbrains linters are proprietary, though they can integrate Open Source ones, and generally better and more thorough than Open Source ones. They're especially better for professional development where you use various frameworks.

> I take offense to it when part of the onboarding experience is setting up an IDE and someone really pushes it on me.

That's fine, but being a part of a team sometimes requires sacrifices. It's as true in football as it is in software engineering :-)

Plus I'm sure reasonable teams just leave you alone if you do your job well and your setup doesn't intrude on what everyone else is doing.