This is both annoying and understandable. For the dev team, to dive into this performance optimization would mean punting something else from their roadmap. I suspect they weighed their perceived chances of success against the projected cost and reached a certain conclusion which in retrospect seems wrong. The independent developer did not have that "hurdle" as he simply did this at the expense of whatever other fun programming challenges he may have done those weekends. It's awesome that he turned out to be right. But I suspect if he turned out wrong we'd never hear about it. There was likely no tradeoff analysis on his part in undertaking this project, while the dev team did have to do one.
On the flip side, I have seen developers get really hung up on what is academically difficult rather than what is practically doable. I had a young developer who worked for me get really upset and hung up on the fact that something he was asked to do was NP equivalent, meaning there's no non-exponential time algorithm to implement what I was asking for. It took me weeks to realize this was messing with him and then about 5 mins to dig into the data and show that input length never practically exceeds like 7, so an exponential algorithm was fine. So I can see how a dev could get stuck thinking something is really hard when it really isn't. But I don't think it makes them a bad dev, it's just hard to transcend your own perspective.