I'm not going to say that's untrue, it obviously isn't. But friction, distance, and drift are going to work in the opposite direction

The result depends on the ratio. I don't see why we can't have trade and relations with other countries without also assimilating eachother

I don't think trade and diplomatic relations is enough. Most people are neither international traders nor diplomats. You need constant cultural exchange by a large sections of the populations for the cultures to drift towards each other.