I know nothing about this particular case, but in general there are many possibilities to get a big investment very early, including, but not limited to:
1. Having "shark founders", who had made a big exit before, so chances are they can do it again (plus, they probably know the investors personally). A friend of mine had made a $200m exit and a couple of years later started another project and got to $150m valuation based on investments long before launching even so much as a prototype.
2. Solving a problem that is close to investor's heart, so it's not only about the money. This often happens when the investors are from the same industry (or the industry itself). In fact, the largest seed investment I know of (Iridium, $4 billion!) was made by the industry. Also, another friend of mine raised $5m from the industry to develop a very interesting piece of hardware, that the industry apparently wanted very much.
3. It is also possible that their actual situation is much more impressive than what you've described, the fact that got lost in retelling (or perhaps they just don't tell everything about it to non-investors).
4. Conversely, it is possible that that's not what really happened, that there is no actual investment. Yet another friend of mine have been telling me that he's got like a dozen investors lining up to invest in him under $5m valuation, and he's in "very advanced stages" with most of them. In reality he met with each of those investors exactly once, they invariably faked interest, promised to call and never have. But he still believes that he's like one technicality away from the money.
5. It is also possible that the investors are idiots. A friend of a friend had managed to get $1m in cash, while having no product, no team, and no idea whatsoever! He just said: "Give me a million, I will hire the best team, we'll come up with a great idea, and just do it". After a year of searching he found some moron in South America who fell for it (or perhaps was too rich to care).