This "do not happen at the same time" is true in plain Xorg as well, though, since the final blit to the screen happens in the X server and not in the application.

Your example of 100fps vs. 130fps on the same screen is inherently unsolvable in a proper way with anything less than a 1300fps display. So you have a bunch of tradeoffs, and I'm sorry to say that if the tradeoff you prefer is tearing, you're in the losing minority by far.

That said, if you truly wanted to write a tearing Wayland compositor, you could easily do so, and in any case plain X is still going to work as well.