Yeah... the THT is an unfortunate contentious issue, there has been a LOT of discussion about the THT going back over time.
Hena decided on underarmouring THTs at 5+ Reinforced in the Scions list so that (as per their background) THTs were used primarily as a transport and not a bomber or as a viable air assault formation on their own when empty. The THT is identically as tough as the Thunderhawk in 40k and SM aircraft are designed to be tough and to survive dropping into hot fire zones and many of us were very against the 5+, but Hena stuck with it.
I agree that it should be focussed as a transport and a straight translation from 40k would make it too good at other things (4 x Twin Heavy Bolters with high FF would be very good for ground attacking or empty air assaulting) but I much prefer the approach of doing the armour properly, so that it can survive and be tough as in it's primary role as it is meant to be, but artificially lowering it's FF and reducing (or ideally entirely removing as per Jimmy's suggestion that others agreed with too) it's ground AP attack to limit this instead. I suspect you copy and pasted the version from the old BT list in the Compendium, which I'd changed after taking on the BT list. The version in the old Compendium wasn't ideal as it changed to 4+ Reinforced for the Templars list only after Hena left, but left the cost unchanged, making it too good.
See
this thread for a lot of discussion from a few months ago on the THT. As discussed there I am testing the following currently in the BT list: Armour 4+ Reinforced FF6+ Heavy Bolter Array 2 x 15cm AA5+ and I will certainly test the unit a lot. I've tested the BT THT in one game recently against Markconz, but I played poorly (rusty on air assault tactics after ground pounding so much) and the game was very one sided and overly quickly so I didn't learn much. For what it's worth Epic-UK originally had the THT at 5+ Reinforced but revised it to 4+ Reinforced a few years ago.