"Los gatos no lavaron sus camisetas ayer."

It continues teaching you preterit while introducing animal vocabulary. Yea they could have used human subjects but then would have had to introduce animals in another lesson. This was a creative way to do both preterit and animal vocab!

Besides that, it could be personification, which is a powerful literary tool. We don't think of The Lion King as such nonsense, yet it does the same thing. If you ever read a story in Spanish, you will want to know how to recognize things like this, especially if it's a children's story.

Duo has had nonsense sentences from the very beginning. How is this one any different?

I didn't like Animal Farm in any case.... These nonsense sentences, together with duo's clipped audio, don't help my listening skills. The lack of logic is very unhelpful at this basic level of listening.