If it is the FSR it probably has a seat-dropper post which you would most
likely need to leave fully elongated while the bag was installed (or swap
a conventional seat post in for the bikepacking configuration with the bag).
Hard to imagine needing to run a seat so low on a hardtail that there wouldn't
be enough room for the bag's wraparound on the seat post (and tire clearance),
but, you never really know until you try