* Survivor bias (https://en.wikipedia.org/wiki/Survivor_bias) is yielding a self-selection in the positive results. I'm sure there are plenty of stupid ideas that went nowhere. Probably as much as "Great" ideas that went nowhere. So, I think saying that his opinion that something was stupid is a counter-indicator of its degree of success doesn't have much merit.
* Second, an idea or early prototype has little correlation to its inevitable success or failure. As is often said, execution is everything. The idea is necessary, but not sufficient to guarantee a market success, as defined differently by different markets.
* Third, while he decries his stance as calling something "stupid" as an arrogant position, this article itself comes across as arrogant -- in effect he's positioning himself near the beginning of these companies' inception and being in the position of being asked his opinion in a taste-maker fashion. I suspect in reality, he was one of many people that were exposed to this idea and his opinion was mostly irrelevant in their efforts to pursue the idea. It can be interpreted as very Forrest Gump to position ones self at the crux of so many eventual market successes when in reality that is not the case. I'd like to understand the context of these meetings. I've certainly been at the early-stages of many ideas, but no one asked for, or cared about, my opinions. In which case, saying that they were stupid ideas was an opinion offered only to myself. And I probably did indeed think they were stupid at the time. Was I arrogant? No. But then I don't claim that anyone cared about my opinion.
* Finally, I call your attention to Pinterest's actual success path (https://en.wikipedia.org/wiki/Pinterest and http://www.famousbloggers.net/history-pinterest.html). They iterated and "failed" for 2 years before becoming an "overnight success". I dare say that perhaps at the time of the meeting, Pinterest's idea and execution were probably stupid, and it took time and relentless iteration to make it a success. In fact, "Silbermann said he personally wrote to the site's first 5,000 users offering his personal phone number and even meeting with some of its users." Which goes to show just how many people's opinions he solicited in the process of building to its current level of success.