I mean presumably if someone has done a similar startup before you, you should be expected to know about what they did and understand why it worked or didn't wo

I mean presumably if someone has done a similar startup before you, you should be expected to know about what they did and understand why it worked or didn't work, and you should have some theory about what you're doing differently or what has changed in the world to make your startup viable now. And presumably you should treat your current competitors similarly. When pg is saying that thinking about competition is a waste of time, he's not saying you should ignore the competition, only that you shouldn't sit there stressing out about it rather than working on your own startup.