It‘s a reference to this historic comment when dropbox was first shown on HN: https://news.ycombinator.com/item?id=9224

This is the (in)famous comment https://news.ycombinator.com/item?id=9224 though, as I wrote, I don't think it's particularly special, it's just the one that gained notoriety. I actually responded to one just this week, they're rife.

The optimism bias is a problem even for programmers.