Tetsuo was extremely angry after hearing what Alex said. The one he threatened and is looking down upon, is now the one looking down on him. This situation was not right, this was all a mess. Not only did Alex not beg for forgiveness or cower in fear, he actually lectured him and his men.
"What are you doing?! Attack! F*cking kill that bastard!" When they heard Tetsuo's screaming voice, the men in front of Alex who got their swords broken, used whatever remained of their swords to attack. When Alex saw the incoming attack he was surprised since he though that these guys were complete amateurs, but it seems that they weren't totally hopeless.
Attacking with a broken weapon is indeed a bad idea, depending on the weapon. But the way Alex sliced the the blade of the sword those two were holding, was still left with a sharp edge that could be used. So when Alex saw that they didn't discard their weapons nor did they panic or flee, he was truly impressed.
