It gave us other meanings of the word. Not the wrong answer. The hints are like a dictionary, they do not tell which meaning is the correct one for the given exercise, only a list of meanings. It is up to us to make our best to try to give a good solution for our exercise. Then we are to reflect upon the result and ask each other and other resources for axplanations and then try again until we master the novelty.
That is an impersonal pronoun which is the clue that "matter" is the correct definition. There is usually someone who imports and that will help determine the definition. Also, to import is a transitive verb. There will be something that is being imported : a direct object.
Could an alternate translation be "why does it matter"? Typically in English when we say "since when does it matter" the answer is usually a reason rather than a time/date.
You are not allowed to talk to people you do not know anymore.
Since when does it matter who I talk to?
Since you started talking to strange men driving white panel vans
The answer is phrased as a time but in reality it is giving a reason for the original statement and is equivalent to "because".
I agree with musekamp. The conjugated verb "importe" (3rd pers singular) yields "is important". Without that, there is no verb in the sentence. With the assumption that there is a verb in that expression somewhere, we can reasonably infer the meaning of it as "to be important" or "to matter".
There's no verb in what you typed. "importe" conjugates (third-person singular) "importer," which, in this case, translates "to be important," "to matter." "important" as an adjective is identical to it's English cognate. It can also be the gerund of "importer," but then you'd have "Since when that being important?" which makes no sense.