Whilst this makes sense, I feel that it is maybe a contraction as "eating" bears no information about whom is doing it; whereas the Greek τρώει specifies that a subject is eating.
I suppose it could be included as a correct answer, but that opens the door to every possible form of each sentence being accepted, which is a lot of work for a language as flexible as English. I think that by being taught the most correct translation for Greek sentence structure, it will help us shape the way we create sentences in Greek, thereby improving our quality of speech.
I don't know if I'm right about the contraction of terms in the English sentence, but I don't mind shaping my English translations to be more true to the Greek meaning.
when learning a new language, I think it is important to think in the structure defined by the language. If you have the English sentence "he plays whilst eating" in your mind, translating that will be harder than "he plays when he eats", as "he plays" maps directly to παίζει, "when" to όταν, and "he eats" to τρώει. The more you think and construct your English sentences in Greek structure, the quicker the translation will happen, and the easier it will be to move away from active translation of English sentences and into original creation of Greek sentences.
In English, ‘when’ isn't the same as ‘while’/‘whilst’. I don't know if Greek distinguishes them, but ‘He plays when he eats.’ would allow him to play after eating every time, while ‘He plays while he eats.’ would require him to do play and eat simultaneously. (This is setting aside replacing ‘he eats’ with ‘eating’; Duolingo allows that with ‘when’, and Greek does not distinguish them.)