Depending on local customs, one may say it with or without the article in this context. Be aware that the article is not required, however. Duolingo has pressed the point of translating the article that we sometimes feel we have to use it in English even when it doesn't produce a natural sounding sentence.
A liaison between Être and a noun/adjective/adverb is an optional liaison; that means pronouncing or not pronouncing the 't' are both correct.
I'm afraid I can't quite agree with you here. "Winter is one season. Spring is another. Summer is a third and autumn is a fourth." Depending on the particular context, one might well use the sentence "Winter is one season."
So, the question stands, is there a particular reason that 'une' here must be translated as 'a' and not 'one'?
The additional context I provided made it more natural to translate the article 'une' as 'one' instead of 'a'. (You seem to still be responding to other commenters use of 'the' in translating l'hiver when you say 'using the article'.)
So if I understand what you are saying - the reason to translate 'une' as 'a' in the sentence << L'hiver est une saison >> is that without more context most people would understand this bare sentence to be a kind of definition of winter << Winter is a season >>. In which case, yes, 'a' is the obvious choice for translating 'une'.
But that would also seem to imply that there would be contexts in which it would be accurate to translate the sentence as << Winter is one season >>. And that was what my original question was trying to get at - though perhaps I failed to state it clearly enough.
Well, the French use articles more than we do, so in English it's "winter" and in French it's "l'hiver". In some contexts, "the winter" might also work. In this sentence, it sounds weird.
Everything cannot be a direct translation, or computer translators would be a lot more functional than they presently are.