It increases. It will be 2 * 11, and so on.

For example, 217 banshees attacking in close proximity deal 4774 damage to a target with -1 defense, which is equal to 217 * 22.

Added 5 minutes later
Nevertheless, this is a weak ability because even with a low attack value, the banshee's damage is significantly reduced compared to a normal attack. Unless the enemy has -99 defense. So, it can slightly weaken the enemy if there is no opportunity or desire to simply attack, kill units with 1 hit point, phantoms, etc.