That's a solid start, but what's your threshold for an isthmus? Does Panama count, or is there only one American continent? Same question for the Sinai.

And if they don't count, Australia's inclusion seems a bit arbitrary, given the massive size difference between it and the next-largest landmass (the Americas are nearly 6x the size).

Furthermore, if isthmuses count, why not mountain ranges? The Urals and Himalayas have historically been at least as important as barriers as any body of water small enough to have a land bridge.

So islands belong to the nearest large landmass? I guess you could do that, but then where's the threshold? Obviously, Africeurasia has to be a continent if you want there to be any continents, and America probably is too, but then is Australia a continent that includes NZ and some other islands, or is it part of Africeurasia? How about Greenland? Are the Diomede Islands really on two different continents? Indonesia would be too, if you count Australia as a continent.

I'll give you that that's geologically consistent while being culturally independent, but it's pretty inconvenient.