It's not unusual for fan-made trailers for films using either intellectual property or names they have no legal right to to be contacted by lawyers of the actual rights holders. It is unusual, however, that the reason for the contact is because the rights holder actually liked what they saw and wants to turn it into a real movie. In the case of 'Clown,' it went a little something like this:
Step 1. Make a trailer for a non-existent horror movie about a man who turns into a murderous wereclown.
Step 2. Insert title cards faking the legitimacy of the movie, including one that claims 'Clown' was made by 'Hostel' director Eli Roth.
Step 3. ???
Step 4. Profit!
The wildcard that is Step 3 is normally when things fall apart, but for 'Clown' creators Jon Watts and Christopher Ford, it actually ended up netting them a meeting with Eli Roth, Cross Creek Pictures president Brian Oliver and Vertebra Films head Steven Prince. Roth, Oliver and Prince not only liked what they saw, but they clearly liked what they heard and so they've agreed to fully finance and produce a feature length version of 'Clown' for Ford to write and Watts to direct. From the press release: