The way I understood it:
WITHOUT variable dt, the game will slow down, but not let the player miss things. WITH variable dt, the game will run at constant speed, but frame skips might make the player miss things
Without variable dt, there is no way to make sure the game runs at the same speed if the display refresh rate is not 60Hz (Which makes this option a no-go for me), UNLESS you use swap_interval = 0, which updates the game on a timer, ignoring vsync (which I believe might cause tearing).
With variable dt, you rely on whatever the driver setting for vsync is and the game will always run at the same speed. Also physics updates might be imprecise due to variable dt.
There is no performance penalty for swap_interval = 0 except your FPS will be fixed at update_frequency.