Mixamo offer a huge amount of animations - that’s what they do. However, they aren’t offered on the marketplace as of yet (perhaps we’ll see more in the future?). You’ll instead need a subscription to their service, in order to download them (there’s a variety of payment plans for different levels). As such, all of them will work with the Mixamo characters. It’s pretty clear they’ve released ‘sample’ on the marketplace to entice people in, really. But, I don’t think there’s anything fundamentally wrong with that.
Guess for the time being, it’s best to wait on further developments on retargetting.