You can invent elaborate conspiracy theories about how Musk is laying to everyone including NASA.
And how SpaceX does pro-bono work for NASA in hopes of ripping them up later.
How NASA, Jeff Bezos, Government Accountability Office as well as courts are blind to it.
(Bezos already dragged NASA trough GAO and courts with regard to HLS)
And Musk is doing that instead of doing it in the honest and straightforward way like Bezos by just lobbing the Congress and asking for a lot of money.
Or you could just accept SpaceX track record on space programs and that SpaceX delivers cheap services.
To do it they need around 6 launches for refueling in orbit.
The very upper limit on a single Starship launch is 0.5 billion.