I’m not familiar with the details of the design flaw and whether or not Wayland fixes it, but those links don’t contradict this being an X-specific design flaw. I get the impression that swaylock is a direct port of i3lock, and thus stands a fair chance of being written and architected in an X style, rather than taking advantage of any superior form that Wayland may support but X didn’t.

Expressed otherwise: just because someone’s written one piece of bad software for Wayland doesn’t mean Wayland doesn’t allow you to write good software. (Whereas I get the impression from what I’m reading that X makes it impossible to write a good screen locker, if by that you require that it be crash-proof and use the usual platform toolkit for the UI.)

(Remember in this that I’m saying I don’t know. I’d like to hear if Wayland does have a good answer to this, or from anyone with definite knowledge that it doesn’t.)

If you've ever looked in the bugtracker of a big X11 screenlocker, you would love to have this small amount of bugs. In fact, some of the bugs you posted are alread solved and I can't find one bug related to displaying. Giving the display to the user could also lie in other code parts. We'll see how this ends, but it's already a huge gain that not every Everyday Linux user has experienced such things by themselves.