It’s surprising because your description can’t possibly be a recipe for generating startup ideas. pg was most passionate about lisp. There must be more to it than what you said.
I try not to dismiss ideas out of hand, but this seems like bad advice for someone explicitly trying to become wealthy. It’s great advice for YC to be giving, because it would maximize YC’s returns over decades: if everyone works on what they’re most passionate about, YC would get a flood of applications from exactly the type of people most likely to be effective founders. But for every passionate founder that chooses to build Stripe, there are dozens who would choose to work on programming languages, or game engines. “Passion project" is practically synonymous with an artist not intending to make money.
Even if today you were most passionate about spreadsheets, would you really choose to build skysheet again?
It’s impossible to imagine pg writing "If I wanted to start a startup, I would work on what I was most passionate about, since it’s the most likely way I’d succeed." The sentence seems mistaken, so pg must have said something more precise.