Yes, seriously underrated, and not just usable by tanks. Non-tank, non-missile units that do a lot of fighting at close range can certainly benefit from it.
I think they meant:
When you defeat Warmech at Lv 40 and Lv 70 you get a Armor of Light +2 (Lv 50) and Armor of Light +4 (Lv.50).
The truth is:
* At Lv 40 you get a +2 Armor of Light (Lv 1)
* At Lv 79 you get a +4 Armor of Light (Lv 1)