I’m going to preface this and say that I don’t use Debian or Sway but I think I can help explain the reddit post a bit. On mobile, please excuse the formatting.
Wayland is a protocol that isn’t responsible for drawing anything to your screen by itself. This job is done by a Wayland compositor. (They’re similar to window managers on an X11 system if that means anything to you)
Sway is one such compositor that Debian supports, but it also supports GNOME and KDE Plasma which have their own compositors and the wiki mentions Weston as well.
It looks like Debian defaults to GNOME, so the sway commands aren’t going to be much help. Wayland uses libinput to handle peripherals so none of the xinput commands are going to be usable.
It’s a little in depth and probably not the best way to do things, but I think I have a solution that might work. Hopefully this can at least get you started, let me know if you have any questions!
Reddit implies that in settings -> keyboard -> shortcuts you can create a shortcut to execute arbitrary commands. You should be able to bind a key to “gsettings set org.gnome.desktop.peripherals.mouse speed 0.0” which will keep your cursor from moving and another with the “0.0” at the end changed to something like “0.5” to set the cursor speed back to something reasonable. This could be done as a shell script to toggle back and forth with one key.
Glad I could clear some things up, sorry I didn’t have a solution that works out of box