This is not based directly on a 70mm print. AFAIK nobody has a complete 70mm print, but certainly we’d be interested if one turned up, particularly for Star Wars (mostly for the audio) and Empire (for the audio and video). What we do have is in-theatre recordings (mono) of the 70mm mix, which tells us what the content differences are, and the fact that the Dolby surround encoded stereo mixes are closely related to the same four-track master used to create the 70mm mix. The 70mm mix was a 4.2 mix, which doesn’t quite align with the 5.1 home video standard, but it’s what we have, and 5.1 represents it well IMO.
So with a little audio archaeology, a little technical whiz-bangery, and quite a lot of audio skill, hairy_hen put this together, which is a reconstruction of the 70mm mix, but not literally the 70mm mix itself. But knowing the care that went into it, I trust it’s pretty close, and it’s my favorite audio track for Star Wars.
Similarly, the 5.1 mixes for Empire and Jedi are based on the same surround encoded stereo mixes being related to the four-track master used for the theatrical six-channel mixes. The difference being that we have no references for the 70mm audio mix for Jedi, and the 70mm mix for Empire requires the 70mm video (it would not sync with the 35mm video, and has unique audio we’d only be able to get from a print), so the six-channel audio we have is much closer to the stereo tracks in terms of content. These are more of tasteful upmixes than reconstructions, although hairy_hen did put some serious effort into improving the LFE channel, so they’re not simple upmixes either.
My SE knowledge is limited, but IIRC the 97 mixes are pretty good, but they introduced a bunch of crap in 2004. Some of those things, like the swapped surrounds, were later fixed. But a lot of the crap remains, and no SE mixes have measured up to the 97 mixes since.