Personally, I think X-1999 it's the best creation of CLAMP, featuring a rather atypical depiction of the Biblical apocalypse from a Japanese perspective.
The 2001 show was a rather decent adaptation of those comics, and this video animation is a basically a complement for that series, being heavily tied to it: For that reason, even when it could be seen as some of sort of short "prequel", it could be a bit confusing to watch for any viewer who doesn't have any familiarity with the source material.
It also could spoil some crucial moments of the series, so I would advice to watch this after seeing the series first for a better enjoyment.
The 2001 show was a rather decent adaptation of those comics, and this video animation is a basically a complement for that series, being heavily tied to it: For that reason, even when it could be seen as some of sort of short "prequel", it could be a bit confusing to watch for any viewer who doesn't have any familiarity with the source material.
It also could spoil some crucial moments of the series, so I would advice to watch this after seeing the series first for a better enjoyment.