They appear to be exempt - they are shown on BRFares as Advance Purchase tickets and not marked as subject to the minimum fare; and various websites will sell them for below £12 even if a pre-10am train is selected.
I would also note that the Heathrow Express website does not ask for the time of travel, does not offer a choice between the full discounted price or £12, and does not contain any indication that they are restricted to travel after 10am - so I would expect that their being exempt is intentional.