As Bull94 already pointed out, the Big 12 playoff candidate this year is already slated to go to the SEC next year. The other potential candidate that would've gotten bumped for Liberty even in a 12-team field is also going to the SEC. Out of the current top 25 in the rankings, the only ones that would have been eligible to make it would have been whoever won the conference out of Arizona (14)/Oklahoma State (20)/Kansas State (25). And going off the 6+6 model and the current rankings, if their champ was #25 Kansas State, they would have been out on account of both the C-USA and AAC champs being ranked higher. (Obviously if Kansas State had won the conference championship, they would have bumped them up over those two). As the current standings go, the Big 12 would have 1 team in the 12-team playoff, and that would potentially take the committee forcefully pushing them over two G5 conference champs.
Will the Big 12 get two teams in here and there? Probably, sure. Is that guaranteed and silly to think otherwise? Absolutely not.