True, I suppose channels 1 and 2 are more summing than mixing, using left and right stereo into the two mono sides of the crossfader, with the crossfader set in the middle. I’m assuming that a mix of fader and panning can get a stereo sum, rather than mono, though I may be wrong there.
EDIT: ignore me, yeah I had early morning brain slowness, channels 1 and 2 are only mono, so that would just be a two channel stereo mixer with channels 1 and 2 as stereo left/right, and the 3rd channel as the stereo mix in.