Because I think Aran'gar has always been shown as in over his head among the upper echelons of the Forsaken, and both times we've seen him, he is only good at threatening or browbeating weaker or smaller opponents.

I actually think that the quote is foreshadowing for Aran'gar's demise - especially since his charge escaped from under his control and he was forced to flee while sending up alerts among the Aes Sedai.

Further indication of his overall lack of impact: while he was busy beating Sheriam for months to control her, it took Mesaana a mere two visits to get concrete results - one to give a task, and the second to remove a finger.