Earlier quoted context omitted.
Is that a counterexample? Suppose they hadn't taken the meeting. What would have prevented Marcelo from starting Care.com? As far as I can tell, nothing. Sittercity had been operating for 5 years in public view. Obvious idea, proven market, easy to find Sittercity users to interview. (However, bringing an EIR to an investment meeting strikes me as incredibly dubious.)
You don't know it's a proven market until you sit down with a potential fundee and talk financials.
Moreover, startup investors are completely happy to invest based on estimations of total available market. It's pretty easy math to look at the number of kids, the average cost of sitters and nannies, and typical consumption of caregiving services. If you can also say, "And these guys have been running for 5 years despite [flaws A-M]," that's just gravy, even if you don't know any of their internal numbers.