I was completely confused by that as well. The wording of the post suggests that a 200w head unit with a 300w amp is 500w… It’s not. It will still be 300w, regardless of the high level input from the head unit. Truth be told, using high level inputs is a severely broken method as the SNR degrades substantially. If you must use it, use it only for subwoofers.
I like that he took the time to make this and I don’t want to be a dick, but the information is hardly spot-on.