When you are the third winningest pitcher in major league history of course you will get into the Hall of Immortals.
Winning 373 games over a 17 year career with a minuscule 2.13 ERA, Mathewson was the first great pitcher of the modern era. In 4 seasons he won 30 or more games, obviously something that will never even be approached again.
Christy Mathewson is clearly an Immortal
For some background on what this is, check out the introduction post here.