I assume their data shows that funny phrases are more likely to be shared by their users, which drives user engagement, brand awareness, and new user signups. Having worked in B2C social-ish tech, Duolingo is well known as having some of the most sophisticated data, measurement, and experimentation systems in the medium-sized tech company space.
Shiiiiit. Everyone in my class failed Spanish because the 4 years I was going to high school, they constantly had Spanish teachers coming and going nearly every month and each time a new one came in they would start the cirriculum over from the very beginning. I had to just ask Spanish speaking friends and co-workers to help me learn it on my own after high school, and even then I'm just barely able to talk to someone who only speaks Spanish.