perhaps they should have termed it "pure metal" instead of "true metal" to sort of like prevent misunderstandings.
true metal refers to metal not tainted by genres not related to metal (ie rap, hiphop, turntables stuff, sometimes hardcore, the list goes on..). and its because these "new" genres with non-metal elements mixed into the music are presented to the mainstream audience as "metal", people often misunderstand bands like linkin park, nickelback and so on as "hardcore metal bands".
and this in a way pisses elitists/purists off cos that is not what metal essentially is.
anyway, i suppose you can read more about true metal on wiki or just google it. the originators of the term is Manowar lol.
the weird thing is how even among "metalheads" there can be conflict over which metal sub-genre is "true metal", with the death and black metal fans calling power metal "false metal", and vice versa.
oh well. elitism is a weird thing huh.
personally i just listen to whatever sounds good to me. the new slipknot album is pretty decent if not for the vocals, and coming from me i bet most of the elitists on metal forums that i am on are gonna condemn me just for that comment.