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.