This isn’t necessarily true. They know that there are unknown unknowns at play here, as nothing quite like FH has ever flown. There is a non-negligible chance of something unforeseen rearing its ugly head, and if you’re just launching a lulz payload, that’s fine.
The Roadster is the PR payload. The real payload is the set of all the sensors & transmitters in the rocket. They'll be getting a lot of engineering data whether it is a successful launch or not.
