The German definition of frenemy (or frienemy), namely that of a "pretend friend", that you found appears to be how many Germans interpret this English portmanteau.
One German blogger even started a blog around this definition!
However, English speakers may also use it to refer to "... someone who really is your friend but is also a rival" (Wikipedia).
You can find this alternative meaning used probably as often in German, too:
Fabio Stassi fesselt in seinem Schachroman beide Rivalen schicksalhaft
aneinander: "Wäre Capablanca nicht auf die Welt gekommen, hätte
Aljechin keinen Rivalen gehabt"; umgekehrt wäre Capablanca nichts ohne
seinen hassgeliebten Freundfeind gewesen. Source: Pfälzischer
So as the quote from Merkur shows, when you are thinking of a person as a frienemy in the sense of someone you love to hate, use Freundfeind. (Make sure, though, to surround it with sufficient explanatory context as it is not a common word.)
Update: According to DWDS, the first writer to use the word may have been Lionel Feuchtwanger (b. 1884 in Munich - d. 1958 in Los Angeles). It is not tied to a specific geographic area. (DWDS lists eleven uses in articles in Die Zeit, a weekly newspaper edited in Hamburg but published nationwide, alone.)