Somehow they've managed to stay out of the news, while Rust has been endorsing BigCo's regularly for a while now...

It’s more the other way around — big cos are endorsing Rust and joining the nascent foundation. It’s perfectly reasonable for a three month old foundation for a popular new language to be in the news quite a bit, and for its sponsors to enjoy the attention that goes with it

The Rust Foundation is really new, it just started in February. That's probably why you've been hearing a lot of news about corporate Rust sponsorships recently, or if not "a lot" then more than you've heard about, say, corporate Python sponsorships.

What does it mean for Rust to be endorsing BigCos?

Not much other than reflecting the reality that BigCos are using rust in production, and as a result, have a vested interest in the project's governance. This is true for pretty much every popular language of software with an open governance model.

It means any Rust dev will be even less likely to open their mouth when Facebook does some horrible shit than what they already are.