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.
I think it's best if sci-fi movies avoid specifying the year in which they take place. Or if they do give the year, just give the last two digits of the year, leaving the exact century to our imaginations, like Cameron did in Aliens
or George Lucas did in THX-1138. THX-1138
leaves it to our imaginations whether it takes place in 1987, 2087, 2187, or some other century even further in the future. It helps to avoid the film feeling awkwardly dated later on, like 1984
or 2001: A Space Odyssey
or all of those references to Khan being from 1996 in Star Trek.