I find this argument unconvincing. The UK is an island, and it's doing badly. "But it's highly connected with the rest of the world, so it makes it honorarily a non-island!" Okay, sure. How about South Korea? It isn't an island, and it's doing pretty well. "But it's de facto an island, it's not getting cases from North Korea!" Okay, sure. But by that standard the US is an island: it didn't get its cases from Mexico or Canada.

And, of course, China has more land borders than any other country in the world, and it's managed to engineer the most drastic turnaround of any country in the world.

Having a small number of entry points is useful to limit initial inoculations, but internal policy and cultural cohesion are key for preventing the explosion of any successful inoculation into national disaster, regardless of a polity's geography.