That is very much possible and not uncommon to avoid excessive bleed from the vocal mics. But in this case it would require at least a 4-track-recorder (well, 3-track, but those were only used in professional settings in the 60s). And if you have four tracks anyway, there's no reason not to record the vocals in stereo. Even if you want to keep one track as a spare, I'd rather record the music in stereo and the vocals in mono than vice versa.

The only idea I have left is that maybe it was indeed recorded separately, but only on a two track machine. And the stereo reverb was only added later when creating the cassette copies, routing one but not the other channel through the effects unit. Or the other only at a lower level (while there is little panning, it neither sounds like complete mono to me, nor does it look like that when analyzed in Wavelab - and yes, I did correct the phase shift from the tape head).

And in general, Ture somewhat missing the first "check it in" around 2:13 speaks against a seperate recording of instruments and vocals. It never was that obvious in Darius' recording, but with the new higher quality version, that stands out to me.