What would make that unlikely? Sounds like a pretty reasonable valuation. Netflix is $400 Billion USD for ~300 million subscribers and is likely an inferior product to a pirate streaming site (otherwise, obviously, the pirate subscribers would go with Netflix). $50 billion for ~20 million users seems ballpark plausible.

Note that black markets exist because there is enugh demand for a product that it overwhelms the ability of the army/police to enforce the law. This site is obviously going to be doing something economically popular. If it wasn't making a killing, ol' mate running the place would go find some other career.