True, if they want to start from scratch.. but google MIGHT not need to. If they just want to develop Linux then they can get a head-start.
What makes you think that Google would want to release an OS based on Linux? That's not typical of Google -- so far, they've managed to innovate quite well by doing things differently.
Look at the people they've taken -- Joe Beda, for instance, used to work on Avalon, which is quite a strong core-infrastructure related area. If Google does anything at all, it would be something subtle and not something that's obvious.
Look at it this way -- Google's arms reach out to searches, newsgroups, emails, shopping and even into a significant number of desktops (in the form of Google toolbar/Picasa).
Who knows, they may come up with a new OS -- that you can remotely boot into off their network, without having to install it on your system. You never know. Even if they do, there is no reason it has to be Linux -- that would entail them to release the code into the pool, which they may not want to at this point. I would say even if they DO choose, it maybe something that resembles a FreeBSD-ish thingy.