All of those has used an existing part of their logo as their favicon.

That's true for McDonald's, but certainly not the other three.

All of them has a favicon which is also a part of their logo.

But all three logos were designed and introduced at the same time as their corresponding favicon. Your claim was that the favicon was just a copy of an existing part of the logo.

Your claim was that a favicon could be a different font than in the logo. But these examples are all a part of the logos. Google would have needed to rebrand their logo to do the same.

The claim was, "Using a different font [on your favicon than the one you use in your wordmark] would make absolutely no sense."

I believe I have successfully refuted that claim.

I think you're mistaking their symbol for "a different font". Each one of those has a logotype and a symbol, which make up their visual identity (the word 'logo' is used interchangeably for those, or their combination). They are merely using the existing symbol as favicon - the stylized N/B/B. Google's symbol at the time was the uppercase G in the same font, so creating a new one for the favicon would be considered a rebranding.