That statement couldn't be more wrong.
Metalcore spawned when hardcore punk bands crossed over with thrash metal, i.e. D.R.I. and The Suicidal Tendencies.
Other than these cross-over bands, the first wave of metalcore consisted of bands like Hatebreed, Integrity, Converge, Shai Hulud, Biohazard, Earth Crisis, and Rorschach. Late 80's - Early 90's.
Poison The Well plays more of a Melodic Metalcore, and definitely did not create the genre.