He was mocking him for clicking instead of using keybinds.
To the OP : Its an odd design element of the fight that its *easier* with fewer people, so pointing at it and saying "herp derp I can solo it y u so bad?" isn't especially clever. most of the p1 deaths are a combination of toxic link splash+whispers of hethiss, a non-factor when you're solo. In p2, its harder to position Hethiss so he doesn't breathe on any of the 5 still spread out people, vs just 1. And in p3 there's no chance of being boxed in by another Bloodvenom targetted on people standing in your path.