TL;DR: F-Droid isn't referring to that, but yes, the app requires an API key for a paid service to perform unlimited requests.

Long answer:

When using the expression "non-free", F-Droid refers to something not being free software, where the term "free" doesn't refer to its price (free as in beer), but to it giving its users freedom to do what they want to with it (free as in speech).

However, this application in particular relies on a service called AudD, which is a paid service based on the number of API requests done. So while the F-Droid "anti-feature" list doesn't refer to its price, this app still relies on a paid service and requires an API key upon launch (although it seems you can do a limited number of requests without one).