Part of what the author is getting at is how we think of buying/conquering/annexing the various island territories as colonial expansion, but westward expansion is not generally thought of that way.

There's a generally unexamined issue in attributing power, wealth, or influence to colonialism, which is "Why was one side strong enough to colonize the other in the first place?"

You might be right in thinking there's a tendency to undercut successful cultures by overly focusing on whatever historical injustices they committed (as though other less successful cultures committed no similar injustices).