I think your simplifying this a bit to much.
In general when a light tank faces a light tank they have roughly equal chance of destroying each other, the BMPTs survivability means that it has (at it’s previous BR’s) a higher chance of surviving an encounter with a light tank and fire back and win.
When a light tank encounters a MBT the MBT has the first shot and a high chance of destroying the light tank in practically most of the places it can hit it, not all, but a lot of the area. If the MBT fails to destroy the light tank then the light tank has a chance to fight back and disable the MBT by shooting the barrel/breach/turret ring/tracks/etc to eventually drive around it and destroy the MBT. When the BMPT face a MBT then the MBT has to treat it like an MBT and aim for weakspots, there is comparatively a much larger chance for the MBT to fail at this due to the MBT style of survivability of the BMPT, at that point the BMPT has even more offensive options than most other light tanks and disabling and destroying the MBT is just as easy or maybe even easier than it contemporaries at it’s previous BR.
I do not think this hinders it in any meaningfull way, it has ATGM that can destroy enemy tanks completely or destroy their barrel/breach so that you can move forward with it easier, other light tanks that face other tanks at a distance face the exact same problem, but they can at least shoot other light tanks down, but not the BMPT to the same extent and the BMPT can fire back much more effectively than a MBT in the same situation.
I do not believe this statement, at all. It’s quite literally an MBT hull. It should theoretically have about the same survivability as MBT’s.
it has ATGM. This is the case for ANY light tank.
For the main canon, sure, but it’s lower penetration values doesn’t hinder it from disabling barrels and things like that, it can also still destroy other light tanks and has fire on the move ATGMs.
That’s the issue, it has MBT survivability so other MBT’s have to aim for weakspots. When the same MBT’s face other light tanks they do not have to do that and have a far lower risk of missing the shot. after that the BMPT has the same, if not better, chances of destroying the MBT compared to other light tanks in the same situation.
It’s not that it’s impossible to destroy, it’s that it survives better than contemporaries and destroys enemies better than contemporaries at those BR’s (10.3-10.7). It quite literally performed A LOT better than the vehicles it faced at the lower BR’s. Otherwise they wouldn’t have increased it’s BR this much in this short of a time. I don’t think i’ve personally seen any vehicle change BR this fast and much before.