Did some testing. Marshall was in full condition.
With katiba could not hit crew from behind or sides. At front I could hit driver and killing him (needed multiple shots), shooting through that little window (in pic red square).
On sides I tried Mk18 7.62mm and could hit (probably) gunner and/or commander. So even 7.62mm will go through on some parts of vehicle.
But in your case if I saw it correctly enemy was shooting from behind, so probably that badly damaged vehicle did not gave enough protection anymore as Blubb said. Good to know.