To see what warrant is, note that not all true beliefs constitute knowledge. You are an ardent Detroit Tigers fan; out of sheer bravado and misplaced loyalty, you believe that they will win the pennant, despite the fact that last year they finished last and during the off season dealt away their best pitcher. As it happens, the Tigers unaccountably do win the pennant, by virtue of an improbably series of amazing flukes. Your belief that they will, obviously, wasn't knowledge; it was more like an incredible lucky guess. (WCB, xi)
WCB was published in 2000, probably written in 1999, and the reference to the Tigers coming in last in the previous season (1998) fits with their 65-97 record and fifth-place AL Central finish that year.
Or, it's possible the passage refers to the god-awful 1996 season (Tigers finish last in everything with a 53-109 record but don't hit the bottom reached by the 2003 team: 43-119). This means that the pitching trade Plantinga refers to could be the one that sent then-closer Gregg Olson to the Astros and brought, you guessed it, Jones (!) to the Tigers.
Needless to say, the Tigers didn't win the pennant in either 1999 or 1997.