I've been searching around, and I haven't come across anything that says you can use DMCA takedowns on anything other than works that you own the copyright to. So, while it might well be illegal under some law to host the code of Popcorn Time, I don't see how a DMCA takedown notice from the MPAA, which does not own the copyright to Popcorn Time, can be valid here. But IANAL, and I also don't know if there is some provision in the DMCA that covers this.