I don't think you could pick a worse example. Space X was founded by a billionaire, who injected 100M USD from his own pocket, and then took 20M from his buddies. That's as far as it can get from what people imagine when they read "bootstrapping".
You could say "I want to build colonies on Mars, I just need 10 Trillion in capital!" or you could say "I want to build colonies on Mars, how can I get there even though I only have 1 / 100,000th of the capital?
The same goes for all these startups that think they need 2 million or 10 million dollars to start. You need skills and $100 to start. Someone mentioned distributed systems as an "unbootstrapable product" which is total baloney. Start by identifying clients that would buy your product once it exists and help them distribute their systems. Build up your team and use your spare cycles to build out the features you think you're going to need but currently can't find anyone to pay you to build them.
If you are still hung up on Space X starting with $100m, let me ask you this. Say Elon had lost it all on Telsa and he was starting with a loan from one of his friends to start Space X. How small would the loan have to be for you to confidently say that Space X would never get people on Mars? To me, the size of the loan only dictates the speed of reaching the ultimate goal.
SpaceX is a perfect example though. First off, of course a company like SpaceX requires a lot of money to do anything, that's the nature of a company that needs to produce and operate manufactured goods, there isn't the same cost floor as with digital goods (which is now practically zero).
More importantly, the goal of SpaceX is fundamentally the colonization of Mars, and the idealized method to do that is a fully reusable interplanetary spacecraft. If SpaceX had tried to build that and only that from their start they would have rapidly ran out of money, even $100 million, and had little to show for it. Instead they bootstrapped their way toward acquiring the capability to design, build, and operate such things while also building a corporation capable of funding such operations. Initially they built a small orbital launch vehicle, just barely at the limit of market feasibility, using very conventional technologies and very conventional designs, applying innovation peace-meal where it made sense. Even then it took them several tries to get a launch vehicle that worked, and it wasn't even very competitive in the launch market. But it proved that SpaceX was capable of building launch vehicles, which enabled them to get grants and contracts to build a launch vehicle that was.
It wasn't a demo or a prototype, the Falcon 1 was a fully functional "MVP". And it wasn't investment funding that got them to the next stage, it was effectively pre-orders for the next generation of their product. Then you look at what they've done since, they've built iteratively, advancing toward their goal, but even now they still haven't gotten everything into place that they need. They're still working on partial reusability, still working on manned spacecraft, still working on clustered core staging, and so on. But even so they have viable products that are bringing in a lot of revenue, with new products coming online soon that will bring in even more revenue. They are still several iterations and many years away from their goal but by the time they get there they'll have the expertise, experience, and revenue to actually make it happen.
As with most endeavors, it's almost always easier and better to get to a big goal iteratively. That's how you build skill, it enables you to sell your earlier iterations to maintain revenue to keep your company alive, and each iteration provides lessons which inform the next and can reshape the grand idea you once had.
Elon Musk was worth $160M and invested $160M in SpaceX, Tesla, and Solar City. His investors gave up on him during the hard times. He chose to risk all of his money instead of giving up.
I'm not arguing against your point, but he wasn't a billionaire when founded SpaceX. He sold PayPal to Ebay for $1.5b, but owned only a fraction of that.
