Of course it isn't authentic. But then, neither is the lack of true discrete channels from the actual 70mm version. This isn't a real 5.1 mix, after all—upmixing of any kind results in crosstalk between channels, because it isn't possible to fully separate a stereo downmix back into its multitrack components. Prologic II gives a more convincing representation of this than other processors, but it in the end it's still a compromise based on limited source material.
It's true that the original Prologic gives a mono surround channel while its successor adds a greater number, but do not assume from this that it would sound closer to the discrete channel version. It does not. Prologic II is capable of panning surround effects between rear channel speakers if the stereo source has been encoded that way, but when playing a track like this, where the surround effects from the original mix are all in mono anyway, they simply get sent equally to both rears. The stereo content in the back is only whatever duplication from the front that isn't fully separated out, which mostly turns out to be the music. All content sent to the rear speakers is delayed in arrival time so that any such correlated signals will be perceived as coming from the front, with the rear version simply coming across as ambience.
The fact that there is any music coming from the surrounds at all is inauthentic to the original, because back then these kinds of films were mixed so that the surround channel contained only sound effects, and the music was entirely confined to the front soundstage. When you listen to the 1997 mix, which mostly comes from the four-track master, you can hear that it is a true discrete version, and this comes across somewhat differently than any upmix from stereo ever could. It actually sounds a bit strange to me because I'm so used to hearing it this way . . . but in that respect it is unquestionably more like how the real 70mm version would have sounded. I did briefly consider trying to use it as a source when putting my mix together, but I quickly realized it was virtually unusable (aside from the LFE) because it had been dynamically compressed to a great degree compared to the 1993 mix, which has virtually no limiting whatsoever. The great number of additional sound effects would have made working with it more trouble than it was really worth, also (and of course there isn't a lossless version of it, either). So I decided that the compromise of sacrificing discrete channels in favor of vastly extended dynamic range was a worthwhile tradeoff.
In any event, what really matters is that it sounds as good as it possibly can given the source materials available, and if it succeeds in that regard then I'm very pleased.