This is a sad day for Gentoo...
I don't think so. So Robbins got a job offer. There is nothing that would prevent him from continuing to work on Gentoo on his off-time. Furthermore, it could be a good thing. Robbins: "I'm helping Microsoft to understand Open Source and community-based projects."
Now, they have done their damnedest to get rid of Linux, and they failed miserably. If they mean what they say, about reconciling themselves to that reality, and mean what they say about wanting to improve inter-operativity between different systems, then that is all for the better, in that standards and protocols will remain open. In the end, it will make it all that much more difficult for MS to lock out non-MS systems.
Of course, it's wise not to fully trust MS, given the past history. You can be sure that Robbins understands this as well, and won't be lead into doing anything that will harm Linux and Open Source.