From what I remember, infantry in these type of transports could be shot down by any decent AoE weapon, being flamethrowers, tank guns and even mortar shots if they happenned to land nearby, a direct hit is quite crippling, yet uncommon. So they're only immune to small weaponry. Anyways it was a long time without seeing in action these transports for me, maybe this has changed.
[...] However, now by default this flag is set to "false", so they behave the same like the US and the WH Halftracks where garrisoned infantry units do not receive damage until the vehicle is destroyed .