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.)

I think they're simply saying that lock screens are hard to get right and that shitting on x11 at every chance you get doesn't help anything.

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.