What should an editor do if the authors have guessed who the reviewers are?

Editors need not respond to author guesses.

Regarding,

We appreciate the very candid critical insights of 2 anonymous reviewers, M. Gompper, and K. Beard.

There is no suggestion that either M. Gompper, K. Beard, or both were anonymous reviewers. Actually, the serial comma suggests they are not. (Even if the comma were absent, I'd perhaps assume authors were unfamiliar with its need.) Similarly,

We do not gratefully thank T. Appourchaux for his useless and very mean comments.

Gives no suggestion that T. Appourchaux was a reviewer.

(Useful) insights and comments are provided beyond the reviewing process and should be acknowledged.

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^{There might be exceptions -- e.g., if authors are claiming reviewer bias, having correctly guessed the reviewer's identity -- but the OP doesn't provide sufficient details to determine whether that's the case here.}

I suspect this is rare enough that there is no "standard procedure".

If I were the editor, I'd ask for #1 to be deleted as the authors presumably cannot really be sure who the reviewers are (unless they are known to have voluntarily revealed their identities).

I'd ask for #2 to be deleted as it is simply in poor taste. If they want to complain publicly about the review, they can do so in the "letters to the editor" column (if they are more polite about it), but it doesn't belong in the paper itself.

If the authors make guesses about the reviewers in the cover letter (not in the manuscript itself), I'd just ignore them, or perhaps say something like "Obviously, I will neither confirm nor deny your guesses."

(Disclaimer: I'm not an editor.)