The funny thing about this myth is that wayland does not even try to support Mixed DPI setups, the only thing it supports is, as you put it, janky scaling. Not that X is any better in the end but at least it has the data available if any application wants to try to do correct Mixed dpi (nobody does)

http://wok.oblomov.eu/tecnologia/mixed-dpi-x11/

So in yet another case of worse is better, wayland has the reputation of supporting mixed DPI environments, but not because it has any support for actual mixed DPI but because it is better at faking it (fractional scaling).