The subpoena, and USA Today's response [1] paints a picture of an incompetent and/or inexperienced FBI agent, who is unaware of existing Justice department guidelines specifically prohibiting her from serving such a subpoena.
Reading between the lines, citing "other methods" is the FBI's way of quietly withdrawing a subpoena that should never have been served.
[1] https://storage.courtlistener.com/recap/gov.uscourts.dcd.231...
> "The subpoena is being withdrawn because intervening investigative developments have rendered it unnecessary," an FBI spokesperson said.
I think this is nothing like a "mea culpa", but instead has absolutely everything to do with managing the establishment of precedents to work in the favor of the FBI whenever possible.