I would argue that the operative paragraph here is actually the one just above the one that you highlighted:
“We believe that Tesla, other companies making electric cars, and the world would all benefit from a common, rapidly-evolving technology platform.”
In other words, he is saying that the benefits (here an open source platform to encourage innovation in the industry as a whole) of making patents available to competitors outweighs the costs (here the loss of a competitive advantage from holding the patents). Also notice that Tesla is listed first, followed by other companies and then the world. This implies that Tesla’s benefit is the most important. The sentence could have been written a lot differently had the world as a whole been the top priority.
So the question that should be answered is whether this cost-benefit rationale applies to SpaceX. If so, Musk should forgo patent protection. If not, he should keep his secrets. I think that is really the only question here.
It seems to me that this rationale does not apply to SpaceX. The reason that Tesla benefits from this open source philosophy is the same reason that SpaceX would not, the market. The market for electric vehicles is huge and on the verge of erupting, so Tesla can afford to give up a little market share in order to spur the industry as a whole. On the other hand, the market for space exploration is much smaller and likely a lot further away from eruption. It would be much harder for SpaceX to give up valuable market share in such a small, young market.
So I don’t think SpaceX should give up patents or trade secrets like Tesla did because the two companies are in very different situations.