Just because Leper Messiah had a similar chord progression that Mustaine supposedly "invented", doesn't mean that he should have a writing credit, Jesus! By that logic, Phantom Lord should give Eddie Van Halen a co-writing credit; listen to Kirk's solo and listen to Ed's solo on You're No Good on VH II. This is a ridiculous argument, as are many of Mustaine's arguments.
Should Anthrax' Strap it On have given Glen Tipton and K.K. Downing writing credit because Dime vamped on one of their riffs in his solo?
Nirvana has always been accused of "ripping off" Killing Joke's "Eighties" riff, but obviously Killing Joke didn't care (or didn't think it was really ripped off).