I always took it to mean Seventeenth - Nineteenth Century style colonizing on both Earth and Vulcan - that just seemed obvious to me at the time.
Yes, of course that's what I always assumed too until yesterday. But anything that seems "obvious" should
always be questioned, examined more closely to see what lies beneath the surface. Looking at Spock's line now, bringing up colonization in the context of the Romulans' possible origins, it does seem to imply that he's suggesting that the Romulans are the
result of that aggressive colonizing period -- since how else would a Vulcan offshoot get to another star system except by colonization? It would be a non sequitur for him to bring up colonization in that discussion if that weren't what he meant. Aggression, sure, but colonization is too specific.
So if that's the case, if that was what the line was meant to convey, then it casts the "like Earth" part in a new light. So much was undefined that early in TOS, so it's entirely possible that Paul Schneider might have envisioned a history of aggressive interstellar colonialism for Earth. There's no way to know now, but it can't be ruled out. They were making all this up as they went, after all. It would've made Trek history interestingly different if it had developed along those lines instead. If, instead of this purely benevolent Federation that had never waged a war except in self-defense, we'd gotten a more ambiguous entity with dark deeds in its past that it had tried to grow beyond but that sometimes came back to bite it.
Come to think of it, that would've been a logical origin for the Prime Directive, wouldn't it? Such a strict rule against cultural imperialism, against intervention even with the best of intentions, is the sort of thing that would logically come into being as a reaction against a history of aggressive colonialism and imperialism that was recognized in retrospect as having done great harm.