It's very hard to know how much the deal actually increases SpaceX market cap, but unless Google exits their SpaceX position soon it doesn't even make much sense as a circular deal.
If you want to understand how companies behave you really need to look at things from the perspective of people making the decisions.
Half of Alphabet's revenue increase last quarter came from marking up unrealized gains in their Anthropic investments.
I'm not saying Alphabet is doing this to juice the share price, but I want to point out that they don't have to sell shares to post banner earnings results and see a 10% jump in share price overnight.