From my perspective, Bitcoin was like a startup in its yearly days (while some would say it still is) but so many startups go bust and bite the dust within a year or so, so amount of risk is extreme in the field.
I would not call bitcoin as a startup because its not so. Its based on some ideology and a method to disrupt the current economy and banking system. Its more like a radical technology. Though some may argue that, that makes it an unique startup.
But isn't that exactly what startups stand for, or at least what they claim? In fact, every startup claims something like that. That they have a thing which is doing to disrupt [add something here] with [add something else here] and revolutionize the world (the latter would count as a new ideology). Without all these promises, often completely outlandish (hype), they can't attract investors money.
Of course, I'm not saying that Bitcoin is all that. But this is what people around it nevertheless do, hype it for short. Do you read the forum really? Apart from that, technically, Bitcoin's development matches the growth model of a typical startup, and the dividends it gives to early investors ("adopters") are comparable.