I just looked up installation of Hardi plank = they still want 6" of clearance, from the ground, if it was an outside application; and 2" if above patio deck boards.

So, with that in mind, I suspect you would still want a pre-treated 2x2 or 2x4 on the concrete floor, before you began the Hardi plank (for moisture reasons) Assuming you might wash the floor, sometimes.