It means that you can't use sway (or any other compositor based on wlroots) with the proprietary nvidia driver. Instead, you should use the nouveau driver.
You have a choice of
A: give up the improved performance and features of the proprietary driver to use i3 with wayland, no longer able to take screenshots or use secondary monitors
You can take screenshots and use secondary monitors on sway, I use four displays at home, including one rotated 90 degrees and one 4K display, to make sure I test each use-case before anything breaks for end-users. The performance of the novueau driver is adequate for most non-gaming workstations.
No, they will require patches to support Wayland. But we have created portable protocols which will allow you to take screenshots on 10+ Wayland compositors and are on track for standardization. Third-party screenshot clients already exist, like grim: