Earlier quoted context omitted.
They still went all in, built a nice community around Proton itself, documented it and supported forks, etc.
Yes, because their business profited from that. It's an example in favour of GPL licenses: the interests of the public and of a for-profit venture aligned to produce value for both.
Is Valve still just a company with profit as its main motive? Yeah, that's the point. But it can do helpful things as a side-effect.