If they truly wanted the capex, this would not be a mixed round A fully internal recap would have been simpler. The presence of outside capital, even minority, is consistent with a gradual transition toward shared ownership, asset light structures, or operator partners.
They have made many comments over the years about this too.
Notice I left a list of potential reasons. Not that ownership has changed. Just pointing out for folks like yourself that Google has made commentary about this exploring the idea of partnering with companies that operate the physical fleet. $3bn even if chump change for you is still a larger placement and has some level of signaling indicating the want to get other folks involved at some level.
What gradual transition? Alphabet's ownership percentage is unchanged.