It's almost like he is saying the goal of a power supply is to I-S-O-L-A-T-E the AC from the audio .... His analogy is a bit better when it is all there huh .... Of course if you don't understand what he means, you could come to the conclusion that matches what you want it to say.
When you understand how an amplifier works—that you're making a copy of the signal by modulating the power supply—then you see that the power supply directly drives the speaker. Anything that's not right about the power supply is going straight into the speakers. That's why we use such a huge power supply—take off the lid and you'll see that about 90% of the innards are the power supply. We have a giant transformer, stacks of chokes, and all those filter capacitors to regulate the front-end. Compared to the audio circuitry, whether you're looking at board-space or cost, that stuff represents 80-90% of the design.