Back in my day there were only around 20 pokémon, and we all were Rock type.

Is he trying to tell us that the prehistoric era only contained 150 Pokemon species, or is he denying his own existence?

Well rather than looking at it from a meta standpoint. When you look at the other Fossil pokemon and think about how some of the original 150 were probably "Evolved"(that sounds weird to say) versions of prehistoric pokemon. You could still say there were 150 "back in the day" before things starting going extinct or new species started evolving.

Back in our days, we didn't have battles, we had beatings.