He didn't invent Metallica, but he's a main reason they ever got signed to begin with. He was the most talented member of the band and driving force when they were becoming a popular underground group.
And just a little fact check...
"During his short time in Metallica, Dave Mustaine toured with the band, co-wrote four songs which appeared on
Kill Em All, and co-wrote two songs which would eventually appear on
Ride the Lightning. Mustaine has also made independently unverified claims to have written parts of "Leper Messiah" from
Master of Puppets.
[5] He also recorded several songs with the band including the
No Life 'Til Leather demo tape."