Die Eule will often give credit for typos, so long as the misspelled word is not a valid word. Combine that with Duo's leniency regarding umlauts (i.e., equating a and ä, o and ö, u and ü), and you have your explanation: Landen is not a word, so die Eule thinks you simple had a typo, an accidental insertion of the first "n".
Had you typed "Lade"--which could be interpreted as the opposite typo, the accidental omission of the "n"--then you likely would have been marked wrong, because Lade is a legitimate German word (meaning "drawer").
Personally, I think Duo should be more strict regarding umlauts. That would have probably meant your "Länden" would be wrong, because it would have two typos.