I'm missing something -- both the English and the Irish given by Duolingo are (as sometimes happens) 100% correct.
Irish often uses the definite article where English does not. Different languages, different ways of saying things.
Saying /Tá geimhreadh ag teacht/ would be incorrect, as direct translations between languages often are.
OK, let me try again. an = the, BUT they are NOT used in exactly the same way in Irish and English.
For instance, in Irish you say 'sa Spáinn' for 'in Spain.' Literally, sa = i+an (in the).
Look up 'Monday' on focloir.ie. How do you say 'Today is Monday'? You say 'Inniu an Luan.'
Look up 'prison.' How do you say 'in prison'? You say 'sa (i+an) phríosúin'
There you have three examples of how Irish sometimes uses 'an' in places where English would not use 'the.'
The Irish sentence above and its English translation are correct as given. You have to say 'an' in the Irish sentence even though you don't have 'the' in the English one.
Again, I don't want to be mean, but the sentences at the top of the page are correct and a good examples of these special case. You need to learn them instead of trying to prove that they're wrong.
I am in Ireland at the moment, typing this answer on my phone, so I'm not going to argue or look up answers, sorry.