It's the same as saying that introverts will be always inferior to extroverts. If i managed to snatch a big chuck of territory and instead of messing with wars\diplomacy and further attempts to expand outwards, start to expand inwards, why i should be weaker?
But if game doesn't give you the ability to obtain enough territory peacefully, that's a completely different story.
Thing is, its not really possible (currently) to snatch that much territory that you'd be permanently ahead of the arms race and thus would deter invasion. Also peaceful acquisition of territory isn't what I would call isolationist either, its expansionism through different means. When we say isolationism should be harder to pursue, we mean it shouldn't be possible to become 100% able to deter any invasion into your territory just by building up defenses while having no allies and remaining independent diplomatically. That doesn't mean it shouldn't be a viable strategy, but should come with downsides to it like all of them do.