I have to say "No," based on the generally accepted definition of "diploma mill." Size doesn't matter.
If you want a history lesson keep reading:
In the interest of (mostly) full disclosure, I'm in the "higher ed business." I am not currently faculty, but have taught before.
We have our liberal arts brethren/friends in Austin to thank for the size A&M is today. Last century they had a law school applicant-turned plaintiff named Cheryl Hopwood who filed suit against the university claiming reverse discrimination.
https://ballotpedia.org/Hopwood_v._TexasShe lost, but appealed to the 5th Circuit which overturned the lower court's decision thereby disallowing race, even in conjunction with other criteria, for admission decisions. In response to the 5th Circuit's ruling and as a way to skirt "race based" factors, the state established the Top 10% Rule. In a nutshell, any student that graduates in the Top 10% of their class from any accredited high school in Texas is guaranteed admission to any public institution of higher education regardless of their GPA, extracurricular activities, etc. The intent was to encourage, by removing admission barriers, enrollment into the flagships (A&M et al) by students from any background that might not otherwise meet SAT/ACT and/or GPA admission criteria.
It worked. Too well, in fact. I saw this coming and predicted that it wouldn't last as it was written for the simple fact that in order to defeat this statute the "only" thing that needed to happen was for too many (at the time I said "all") of a particular graduating class to arbitrarily decide to attend one school. My example was some place like Stephen F. Austin or Angelo State. By law, ALL applicants MUST be let in, but what would a school like that do with all of them?
Fast forward to more recent times... The 40 acres in Austin is landlocked. It took a while, but eventually they had to let in more freshman than they could accommodate. That's why they got it changed to Top 8%, then Top 7% and lately the Top 6% Rule for themselves only. The t-shirt parents all over the state finally broke them. A&M, on the other hand, still has room to grow and so it does.
That's not to say A&M is anyone's second choice. It simply that the campus and, to a slightly lesser degree, Lubbock was able to handle the growth.
Here's an fun fact to file under the Law of Unintended Consequences- About 5 years after Top 10% went into effect a "reverse white-flight" took place whereby parents that lived in the suburbs would get transfers for their Top 15-20% kids to "less competitive" high schools for their senior year so they could get that guaranteed acceptance upon graduation. Make a rule and someone immediately starts looking for a loophole or work-around.