I don't think there was ever a design to be one frame behind.

Compositing requires the GPU to do some extra work to draw the frame to be presented. This typically takes very little time (much less than a full frame period). Additionally, most wayland compositors will bypass that extra step if an application is full screen (wlroots calls it "direct scanout").

Also some wayland compositors keep track of timing and delay the final composition until right before it is time to present the frame in order to reduce latency.

Tearing can theoretically improve latency for the part of the screen that's below the tear, but in any case where you could actually benefit from it the difference would be at least order of magnitude smaller than the duration of one frame.