no idea. The article just says:
He said that staff followed information from IATA and that this was double-checked with the immigration office at Frankfurt Airport.
I would hope it is the Bundespolizei they are referring to.
Edited to add: whoever the gate agent checked with, they were obviously wrong as well. I guess the problem is not all British passports allow entry to Germany up until the expiry date - it depends on the category code which shows citizenship.