It actually seems like a pretty good understanding of Musk's track record to me.
They're obviously not going to go out of business as a result. But considering Tesla has already had earnings calls pretty much exactly like that with respect to slower than expected Model S production, yes I would expect Tesla in 2018 to admit that most people on the pre-order list are going to be waiting a very long time.
That leaves Tesla in the awkward position of either needing to massively ramp up every aspect of its production pipeline now (which is a HUGE undertaking) at the cost of not producing Model S and Model X on schedule, or facing a significant number of pre-order people giving up on waiting, and a constrained production capacity well into 2020.
The OP's comment was with respect to shareholder confidence, which is currently propping up the stock price based on future expectations. I believe the point was that if Tesla can't come even close to meeting the market demand for its vehicles, that's a signal of weakness on several fronts that would need to be priced into the stock.
I'm also not sure what landing rockets has to do with mass-producing cars. SpaceX's achievements are indeed amazing and I watch all of their launches, but figuring out how to land a first stage is an engineering challenge. Figuring out how to produce 300,000 cars in a short time is a logistics challenge. Not to mention no one yet knows how Tesla's model will scale to the larger market with respect to service, support, parts replacement, etc. Scale in the automotive market just brings with it a huge number of issues to tackle that don't exist in smaller volumes.