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Rule 15 -- Amended and Supplemental Pleadings.
(a)(1)(A) -- A party may amend its pleading once as a matter of course within: 21 days after serving it, OR...

The relevant chapter in Glannon's Guide (learning through mc and analysis) says: "Rule 15(a)(1)(A) authorizes a party to amend once as a matter of course before a responsive pleading is served, if a responsive pleading is permitted.

Is the Glannon's Guide outdated or did they revise the rule?

The rule was changed in 2009. If you have a supplement look at the committee notes (if they are included). IIRC that was all that was changed. Glannons is out of date on that.