Impossible. On average, the Stuka AT planes deal about 250-500 damage per pass (both planes combined). And that is on a stationary target. If the target moves, the Stukas will usually miss more (but that depends on their angle of approach).
They both need to be hitting the IS-2's rear armor to deal that maximum of ~500dmg, because they have 180/170/160 penetration (IS-2 armor is 375) and deal less damage on deflection, which is highly unlikely on the first pass given that the planes start on opposite ends.
Since the IS-2's health is 1080, that means they can deal a maximum of 25%-50% damage to it. With 50% being the most ideal circumstances, only possible on a second or third pass, and with 25% being the average damage it deals on the first pass. And again, tested on a stationary IS-2, so expect damage to be even less if it's moving.
So TLDR, unless a replay shows otherwise, Sector Assault can not have done more than 50% damage in a single run to your IS-2 and something else must have damaged it at the same time or circumstances were not as you presented them.
The ability requires ground forces to spot enemies or it will not target anything.
Here is a IS2 taken below 50% by 2 passes, the schwer does a very minor contribution, it looks around 30% health after these two passes, I highly doubt the schwer contributed 20%..
https://clips.twitch.tv/RefinedStupidAsteriskTriHard
This ability almost always deals maximum damage within the first 10 seconds and rarely does limited damage. It probably is OP.
The math does not add up. Something is funky with this ability.