If you want to be really picky, a mono downmix from stereo isn't the same as a mix that is made for mono from scratch. The levels are never the same. Which is why they didn't just pull a mono version from the stereo master but mixed it from scratch all over again. Fudging the discreet stereo channels into a single channel isn't the same as making a mix with one channel in mind IMO, although I'm sure this is done all the time simply because it's easier. Back in 1977 the mono mix was arguably the more important one since most theatres weren't stereo equipped, so they spent a lot of time on it.