Where in your quote do you mention quality of opponent? No where.
It should be a given that beating power 5 conference teams means more than beating non-power 5 conference teams and FCS/1AA teams... although I guess it is not for A&M and the SEC based on their miserable out of conference scheduling.
A&M non power 5 non-conference wins while in the SEC:
2012: 4 (Louisiana Tech, SMU, South Carolina State and Sam Houston State)
Notes: TWO FCS TEAMS!
2013: 4 (Rice, Sam Houston State, SMU and UTEP)
2014: 4 (Lamar, Rice, SMU and Louisiana-Monroe)
2015: 3 (Ball State, Nevada and Western Carolina)
2016: 3 (Prairie View A&M, New Mexico State and UTSA)
2017: 3 (Nicholls State, Louisiana and New Mexico)
2018: 3 (Northwestern State, Louisiana-Monroe and UAB)
That is 24 wins against non-power 5 opponents! 8 of those wins are against FCS teams. I mean, Lamar, Prairie View and Western Carolina? Really? Do season ticket holders really want to see that poop?
That same period UT has played 0 FCS teams and the following non-power 5 opponents:
2012: 2 (Wyoming and New Mexico)
2013: 2 (New Mexico State and BYU*)
2014: 2 (North Texas and BYU*)
2015: 1 (Rice)
2016: 1 (UTEP)
2017: 1 (San Jose State)
2018: 1 (Tulsa)
*I do not usually include BYU as non-power 5 and personally usually group them with Notre Dame as a "power independent", but in this case I will consider them non-power 5 as it does not really make a poop.
That's 10 total games against non-power 5 opponents and 8 wins. UT has played 10-11 power 5 teams in the regular season this entire time period.
Meanwhile, A&M has been playing pansy schedules of 8 home games a year and sometimes only 8-9 power 5 opponents in the regular season.
In short, A&M has inflated win/loss records due to cowardly scheduling. A&M is actually playing an easier schedule since joining the SEC.
And as for the Big 12 being a significant step up from the Mountain West, shouldn't TCU have become a middle of the road program in the Big 12 if that were the case? Instead, TCU has thrived in the Big 12. Maybe the Mountain West and the Big 12 are basically the same.
Actually, some programs and, more specifically head coaches, will thrive regardless of conference affiliation. As you said, TCU is not dominating like they were in the Mountain West, but they are continuing to have success.
A&M continues to be mediocre regardless of conference.