The English words 'The bananas are cooked' are ambiguous. As a standalone sentence, it means that they have been cooked, which, from what I understand, should take the past or perfective tense in Swahili. But imagine somebody saying "The bananas are cooked in this manner" before demonstrating a certain method of cooking a certain type of banana. I believe that's the sense of the Swahili sentence.
either: "Bananas are cooked" - i.e. they generically are for cooking (well, not all are),
or: "The bananas are being cooked" - i.e. the ones being expected are.
"The bananas are cooked" - i.e. the ones expected are ready; or
"Bananas are cooked" - ready, some bananas are.
Context clarifies the first and last connotations of " bananas are cooked."