What is meant here is probably that archers don't fire into a crowded melee. That means when your fellow soldiers who are part of the infantry start fighting in close combat your archers generally don't want to fire off into the melee because that risks friendly casualties. Hostile casualties usually would not be great enough to justify sacrificing those friendly casualties. Of course, there are exceptions, but for most sane tacticians, having the archers fire into a melee when your infantry was engaged was probably not the desired tactic.
However, in the case explained by Roland, it appears to be about archers in isolation, or without any serious melee in front of them. Of course, if the enemy is charging unhindered, go fire into them. The current system in BM does have archers fire at enemy forces that aren't engaged in melee with your forces. The current system also has archers fight in close combat even. They aren't very good at it, but they do in fact have the ability to fight in melee.
What people were talking about was the inability of archers to fire into the melee. This might be true for mixed infantry and special forces as well.
