Per my understanding of the rules, STX and STU are different only on displacement limitations and wheel and tire sizing. In the rule book, both the high flow cat and brake swap allowances read both as STX and STU. So unless the group you run with bumped you due to lack of people in class, I see no reason you couldn’t run in STX.
Parking Lot Hero
2008 WRX daily driver
2003 ZX3 23D Autox Car 42 FSP
"Hella Functional - Raced, Not Hard Parked"