It's not exactly a "stunt". The vehicles aren't expected until 2021 at the earliest, but it's not vaporware.
There are over 250,000 pre-orders, but for just $100. Many of those are aspirational and won't actually turn into orders, but it demonstrates that a lot of people are interested enough to put money on the line, and many more people are interested but not willing to reserve a spot in line. Since it will take quite some time for them to produce 250,000 Cybertrucks, the pre-orders suffice to suggest that they can sell pretty much all they can produce for the first few years at least.
There are over 250,000 pre-orders, but for just $100. Many of those are aspirational and won't actually turn into orders, but it demonstrates that a lot of people are interested enough to put money on the line, and many more people are interested but not willing to reserve a spot in line. Since it will take quite some time for them to produce 250,000 Cybertrucks, the pre-orders suffice to suggest that they can sell pretty much all they can produce for the first few years at least.