And there are two post, rich with examples, of the five different situations when in Polish the genitive should be used:

https://blogs.transparent.com/polish/genitive-case-part-2/ https://blogs.transparent.com/polish/genitive-case-part-3/

I was wrestling with this very sentence on the mobile app and lost a lot of health over it-- I thought it was a singular/plural mistake. It now makes sense. That being said, it would be great if the mobile app had this kind of resource because there are no explanations given.