It hasn't been delivered yet, though. At the moment Roadster in 2020 is the promise. Not the delivery.
Tesla used to have a reputation of underpromise and overdeliver sometime back in 2014 when it came e.g. to performance. Unforunately P85D, AP1, P90DL, AP2 have switched over to overpromise and underdeliver since then. Not to mention Elon's timelines in general.
Even if you have a positive outlook on Elon, you can't really say he underpromises and overdelivers. He may deliver great things with these companies, sure, but usually that has been preceded by great hype and possible overpromises at the very least on schedule - not preceded by underpromise.
I think that logic is flawed, as though the standard measure of Elon is whether he underpromises or overdelivers, but he's not interested in any of that.... He's too busy doing things. I'm sure he has his fair share of issues, but I make no mistake, the guy is force of nature in terms of actual real world impact he is going to have. He may end up being the most important visionary we've seen since Einstein. Time will tell, but I have to say, thus far, I don't see anyone like him. Whether we feel entitled to judge his statements through the lens of a automobile customer is the argument your having. Which is a fine argument, but not a particularly interesting one. Not as long as he's staying leagues ahead of other automakers who have straight up inferior products.