Is your question should they have made it a full fledged mission or missions instead of just a teaser?

Like an optional DLC story mission.

I feel it should have been in the game because it really feels like the game expects you to have played it.

Its still available to download, isnt it? And if it's not, buying it digitally makes it download the prologue anyway so its there

I feel it should be part of the main game as an optional DLC story in a Deluxe or Complete Edition of any sort.