Translation:His cat ate your banana.
☺ If the cat ate the banana, did the monkey eat the fish? I've heard of a fruit bat, but not a fruit cat...
This article may be relevant: https://www.chewy.com/petcentral/nutrition-pet-diet-tips-can-cats-eat-bananas/
I think you are correct for the first part, though I have seen cats eat some strange things. But the sentence itself seems to be for practice; a sentence involving food and eating (the topic is dining after all) and is grammatically correct. Honestly, I laughed when I first read it.