The rules for vehicle sqadrons (p79) tell us to allocate hits rather than wounds (as an equivalent to allocating against a normal non-vehicle unit), but otherwise following the normal allocation rules - the nearest visible model to the unit causing the hits.
So if the Falcon closest to the Purifiers would be hit in the side, the hits are allocated there first. As Cleansing Flame doesn't have sufficient strength to harm the side armour, all of the potential hits against the unit are wasted.
To give an inacurate analogy, imagine a unit with no armour saves, joined by an IC with a 2++ re-rollable save - if Tanky McTank is the closest model, it doesn't matter that you'd tear through the rest of the unit, wounds get allocated to him.
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