I assume that they mean batteries are not a net "source" of electricity, they only store electricity that has been produced in another way earlier. Like a farmer scoffing at someone who says they get their eggs from the supermarket, it's not like the supermarket produced the eggs.

GP could have been less of an asshole in the way they expressed themselves though.

I see, that is a bit ironic. He quoted it but I guess he didn't understand the meaning of secondary energy source.

Well he did express enjoyment about learning the topic, so probably he's on the right track.