So, “matrixed stereo” is a stereo mix encoded in a special fashion where, if you pass it through a proper Dolby ProLogic decoder, you can get four-channel audio. The Laserdisc audio can be decoded in this way to get true multichannel. The six-channel reconstructions are based on this four-channel decode of Laserdisc audio, and for the past several releases has used a proper professional-grade decoder for that work. We also have in-theater recordings of the six-channel mix, which aren’t an audio source per se, but allow us to know which slight variations of sound effects happened, and when. The LFE is a bit of a best-guess, using some SE when it aligns with theatrical effects.
This work is all based on the theory that the six-channel and stereo mixes were based on the same four-track master audio, which has at this point been proven enough through various audio discoveries to be considered fact. There’s a lot of archaeology in this as well…
Keep in mind that, at the time, only the Star Wars six-channel mix even attempts to match any unique 70mm six-channel content. That may change, but right now Empire and Jedi are more like “tasteful and respectful upmixes” than 70mm mixes – although they may end up being extremely similar to their respective 70mm mixes, due to the shared lineage of the four-channel master.
ProLogic decoding can’t get you literally back to the discrete six-channels on a 70mm print. For one thing, there’s still a bit of channel bleedover that would not be present on a 70mm print, the other is of course the missing two LFE channels*. But these things were done with love. It’s not at all like those Blu-ray mono-to-5.1 upmixes which kill the soul of the original audio. It can be done well, and in this case, it’s been done in an exemplary fashion.
* The original 70mm six-channel audio was 4.2, not 5.1. It was L, C, R, and a single surround, plus two LFE channels. So a four-channel ProLogic decode gives you everything except the LFE. Since 5.1 is far more compatible with modern playback systems, that’s how it’s encoded for these preservations.