I have a lot if trouble accepting Duo's 'correct' answers when it comes to 'this' and 'that'. I am no master of Polish but you don't hear a lot of 'tamta/ten/to' in Poland unless it's said for strong emphasis. Of course that's my observation, and I may be totally and utterly wrong. For example, 'Where is that page?' I can imagine a person searching a book for the proper page and saying, 'Gdzie jest ta strona?' And I imagine it could convey a similar meaning to, 'Gdzie jest tamta strona'. Am I right? It comes up endlessly in these translations. I think Duo is being too strict, even to the point of being unnatural.
Well, basically, you're right. There's no much need to use 'tamten' and its forms without the need for special emphasis, or usually - contrast between 'this' and 'that'.
And in this example, I don't see a problem with "that page". I think the 'this/that' problem was noticed at an a bit later stage and not every sentence has it already accepted although it should be. So if that was the case here, you can report it.