I don't think I'd ever noticed this. Flipping between the system font option and Proxima Nova, there's not a huge difference (on a Mac). I do find it subtly easier to read however, so would agree that attention to detail is neat.

Yes, subtle difference. They didn't have to do it, but they did. That's the kind of thing that should be appreciated.

What is a "professional font"?

A font that they've licensed for use in their web app. It makes the web app look quite professional in a subtle way.