Its original use was apparently in the British theatre. I've seen it used for those who fund political projects, for example, in referring to rock stars such as Bono. In the case of actual investors in startups, a advice component could certainly come into play. Financial return is not essential to the concept however.
At any rate, I see no problem at all in calling our major financial supporters "angels" (Well, not really, obviously there is a religious connotation which might be inappropriate)
Yeah, it's close enough. I think "benefactors" is better, though.