I don't see why he wouldn't take an AWP to B. You're exposed to less angle and the terrorist have to either walk through a long hallway towards you (apartments) or catwalk to B which exposes them to window and connector.

Plus it's easier to try and take back A with an AWP (if you can't find a better weapon), because you can look to A from great distances (catwalk/mid-B area, jungle, CT spawn).

Whilst you could go B i think he meant it's more beneficial going A or Mid.

Generally you want to AWP over the longest range to maximise your advantage.