Here are the instances since I watched a Texas Rangers baseball game for the first time ever in 1991 that the Rangers have been at least 12-8 through 20 games:
1996: (13-7) - AL West champs
1997: (12-8) - Disappointing third place
1998: (13-7) - AL West champs
2004: (12-8) - In the race until the last week
2012: (15-5) - Wild Card
2013: (13-7) - Game 163
2014: (12-8) - LOL
With the win today, these Rangers join that club. As you can see, if for no other reason than that 2014 team being among them, going at least 12-8 through 20 games doesn’t guarantee anything.