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.