Short answer: There wasn't one in the modern searchable record before Texas A&M's 11/15/2025 rally.
What I checked: every major recap and stats thread (ESPN, Sports Illustrated, SEC Sports, SI/On3 reporting) specifically noted that SEC teams were 0-286 since 2004 when trailing by 27+ points i.e., no SEC team had done it between 2004 and the A&M game. Those pieces also don't turn up an earlier (pre-2004) SEC example in their lists of big comebacks.
What that means: either (A) an SEC team last accomplished a 27+-point comeback before 2004 (but it isn't showing up in the readily searchable modern databases and headline lists), or (B) it simply didn't happen in the modern era until Texas A&M's 2025 win. I wasn't able to find a documented pre-2004 SEC comeback of 27+ points in the sources above.
So has it ever happened or did they pull this stat out of their ass?
"Give me an army of West Point graduates and I'll win a battle. Give me a handful of Texas Aggies, and I'll win the war.”
- General George S. Patton