" How the heck can this planet be five times the mass of Jupiter and not start fusing it's lighter elements under it's own extreme gravitational pressure? "
The smallest true star known (with active fusion) has 100 times the mass of Jupiter.
Arthur Clarke's premise in turning Jupiter into a star was to artificially compress the matter to achieve a fusion process. It is not based on the natural mass and gravitational pressure in Jupiter, but a science fiction construct of an advanced civilization.
I was going to say about 80 Jovian masses, but we're both in the same ball park. Basically, to be a naturally formed star whose energy is being supplied by ongoing fusion reactions, you need a minimum of about 0.1 solar masses. Brown dwarfs are what I like to call "stellar wannabes" who got "voted off the island" because they don't quite have the mass it takes to get the job done of initiating AND maintaining an ongoing fusion reaction.