My guess is that it is an expansion to Far Cry 5 rather than a full-fledged game
Shinobi, an insider and one of the admins on ResetEra, confirmed it is a full game. That matches with the roughly 1.5 year release schedule that the Far Cry series has been on the last few years:
- Holiday 2014- Far Cry 4
- February 2016- Far Cry Primal (14 month gap)
- March 2018- Far Cry 5 (25 month gap, but was originally scheduled for a Fall 2017 release before getting delayed, which would have given it about an 18 month gap)
- Holiday 2019- Far Cry post-apocalyptic spinoff (~18 month gap)
That being said, it appears that it will re-use the Far Cry 5 map, in much the same way that Primal re-used the Far Cry 4 map (the shape and terrain of the map). But it will likely be a full priced $60 game like Primal was, rather than a $20 standalone DLC like Far Cry 3: Blood Dragon was.