So has it always been known that Alien takes place in 2122? I was always under the impression the dates were kept pretty vague in the Alien universe, but clearly I wasn't paying enough attention.
In "Aliens", Ripley makes mentions that Burke sent the message to the LV-426 on I believe 06/12/79. So taking the year as being 2179, and Ripley having been frozen for 57 years, the events of "Alien" take place in the year 2122. So it does seems Scott is paying some respect to what canon Cameron established in the series.