defensive posture

[BigDuke66]

Likely, the rules for skirmishers are that they take just 1/5 of the normal damage if the stacking in their hex is below 1/8 of the max stacking limit, obviously with a formed battalion in the same hex you are likely above the 1/8 mark and by that the skirmishers take normal damage.
If and how the damage in a melee is split between the units in a hex is unclear to me, but I guess the damage is proportional to the size of the unit.