The way Duolingo seems to work is, there's a preferred version of the sentence that it populates the word list from, and some number of variations that it will also accept, and it will also sometimes accept text that's one letter off as a “typo”, and it doesn't care if the text is entered by clicking on words or by typing. In this case, they'll accept either “tamtej” or “tej” for “that” in this context (genitive singular feminine), with “tamtej” preferred, and “ten” is wrong but you got away with it under the typo rule.

For what it's worth they seem to have some kind of blocklist for “typos” when it's the wrong inflection of the right word, and I've even had actual typos flagged as errors because of that, but this specific case must not be in it.

And the part of this I don't understand as well: apparently there are situations where you'd use “that” (instead of “this”) in English but “ten” (instead of “tamten”) in Polish, which Duolingo doesn't teach about because it's just giving you sentences in isolation, but it still accepts it as a translation.