They should definitely be the true 70mm Dolby mixes, haven't read any articles about the creation behind this set and I don't own this set myself yet so I cannot confirm it but isn't it stated on the packaging? That's odd... I would be very surprised if they aren't, IIRC I saw it confirmed somewhere around the time of its release.
The tracks are obviously only available for the theatrical cut of each film, if you do own it, you could easily confirm it as the 70mm mix of Alien does contain several differences compared to the 35mm mix in dialogue, editing and sound effects. For example, Dallas line "Good morning, Mother." is an alternate take than the one heard in the 35mm mix, and when the crew discusses Brett's death, Ash's line "Kane's son" is not heard in the 70mm. I'm not that familiar with the sound mixes of Aliens to know if there's differences in content between those as well.