But they also get a whole bunch of AI Services from Cursor. Other comments have noted that xAI has fallen on bad times (idk one way or the other) so perhaps they were going to spend $5B on getting these services elsewhere, anyway.

SpaceX spending $1B a month on various AI services seems ~plausible

(EDIT - Or maybe it's an IP transfer, or maybe it's over a longer time horizon. Idk but SpaceX clearly expects value from 'our work together' even if they don't exercise.)