If that's the core lesson, it doesn't seem like very helpful advice.

How many resources should I have? Don't have too few resources and don't have too many resources. Have enough resources!

Presumably one of the reasons people have too few or too many because they think that is the right amount of resources at the time.

It is only in retrospect that you can tell whether you have enough.

Viability is tied to survivorship bias, I'd guess. If you didn't survive, you weren't viable.

I felt the book was a bit hand-wavy this way. It all sounds ok, but there did not seem to be a bunch of practical advice.

I would say that should be the going framework regardless. Having come from a failing startup thanks to being lean, I think the lesson I’ve learned is mostly “it depends.” The company I’m at now went on a hiring spree once we landed in YC and got our first few investors. We then ended up having too many engineers, at least for the problems we could reasonably expect people to pay us for, and had to let two people go.