They'd probably use "essen".
Anyway, the distinction isn't as strict as duo wants you to think, I know many people who talk about their pet's food as "Essen". And the other way round people may "fressen", for example when they eat without any table manners or when they're just stuffing their face.
Throughout my personal experience with this app it would seem that Duo considers himself and refers to himself as a functioning member of the elite, what with his holy potato sitting atop his billions and watching senior citizens leap out of windows. Then it would make sense that the sentence would be structured in a way that the" penguin" would eat just like us humans but if they were ordinary run of the mill penguins they would surely"fressen"
When I first learnt German we also had to use 'säufen' for animals and not 'trinken', but this has fallen out of usage. I wonder, likewise, if 'fressen' is going out of use for animals. BTW, I seem to remember 'fressen' being used quite effectively on the factory floor to be derogatory about colleagues, management, politicians, and anybody else that got on your nerves, with the mandatory Saupreiß (Pig Prussian in the native dialect) put in front of the noun. If I wound anybody up, I would get, "Du Saupreiß-Engländer, friss ............!"
The usage of "fressen" is not as uncommon as the usage of "saufen" (in regards to animals), but the distinction isn't that strict anymore. Both can be used for either humans or animals, depending on what you'd like to emphasize. Humans who "fressen und saufen" are still supposed to behave like animals. And animals who "essen" are mostly pets or characters in an animated movie, so beings not as wild as e.g. a real bear.