It's a reference to a quote by Ernest Rutherford. The idea being that every science except physics essentially reduces to categorization, and at the core is physics.

Basically the same idea as this

I guess "stamp collecting" means "finding rare things"? And "physics" is about learning about more complex rules of interaction?

I think that covers chemistry, as we already know the rules pretty well and are just looking for ways to get rare things. Particle physics too, we predict that things exist and many years later finally create ("collect") them.

Ecology and Economics, we don't know all the rules.

Maybe that's a simpler summary: figuring out the rules vs applying the rules.