You raise some good points, but I think the overall caliber of the team that they are building can almost speak for itself. There have been many start-ups which got their start through unusual means. For example, Take a look at this blurb taken from mashable regarding the company Asana (currently valued at an estimated $600 million):
Facebook Co-Founder Dustin Moscovitz and Facebook engineering lead Justin Rosenstein teamed up after leaving Facebook to found Asana, a startup that is reportedly building project management software. It is still in early development and hasn’t launched a product yet. This is one case where the product is presumed to be a sure bet because of the past experience and intellectual caliber of its creators.
Sometimes, a startup can raise money with nothing but pure pedigree. When a handful of big tech company engineers leave the mothership to found a startup, as happens in Silicon Valley from time to time, they can often drum up a round of funding before pencil meets paper.
Over seven months in 2009, the team was able to raise $10.2 million in two rounds of funding. In this case, investors are banking on Moscovitz’s and Rosenstein’s past successes. In a way, it’s as if they are investing in Beethoven's next symphony or Van Gogh's next canvas (i.e. the next Facebook).