I'm not familiar with US legislation, but from my perspective this is just Musk on his soap-box making a statement. It's not something I can rely on while building a business that possibly (!) infringes one of Tesla's patents. They may retract their statement and reconsider at any time, especially when times get tough.
It's nice to hear this, sure, but I fail to see the meat in this announcement.
I think Musk's word in this case is good. He's not a nobody; his reputation is stellar.
Longer term, the other problem is when Musk retires/dies. Of course, that probably wont affect any current patents, but it could to future ones.
Beyond that, anyone who wants something signed and in writing could likely drop a note to Tesla for it when doing their due diligence.
The question is whether its a specific enough statement as to reasonably induce reliance, and whether promissory estoppel would apply. It seems to me (and I'm not an expert) like it may be close enough as to seriously threaten Tesla's ability to successfully prosecute suits, and vague enough that anyone else would still be leery about using anything Tesla has patented.
That would clearly apply in this case.
Sometimes I feel doing business in the US is liking playing with your toys while your mom (courts) are watching. No wonder the legal industry is so huge. It has ingrained in people's mind that other people's word are worth nothing. I wish we could look past that... go talk to the guy (or his whatever-department that handles licensing matters). If you're really interested in starting this business, it won't take much of your time.
Except that the legal enforcement of property rights is something that separates the First World with the rest of the planet, and is an engine for economic growth.