I think a difference has to be made between grounded frames, initial airborne frames, and real airborne frames.
When you jump, it takes 3 to 5 frames to go from jump startup to fully airborne frames : for example, at first frames your character will just push himself into the air but will still have one foot on the ground. Your character is neither fully airbone, neither grounded during those.
Depending on how early you’re being hit, those initial frames wouldn’t get you a reset but a straight combo for the opponent. However, you are airborne, meaning unthrowable to any grounded throw. And that since the very first frame. That how ST works to beat grounded throws : it’s just airborne, even if it’s not fully airborne until the 4th frame.
Past that 4th frame, you would be fully airborne, and get a reset.
Still, some attacks (like Guy’s izuna, Cammy’s hooligan grab, and Claw’s izuna) can grab both grounded and airborne opponents. ST’s airborne status won’t help against those. =)
ST has another interesting property, this being that it has no low hitbox since the 2nd or 3rd frame : Claw is just horizontal, you could kick anything you want it will just pass under him. Meaning it could beat low attacks which would normally frame-trap him (for example cMP after RCF). Plus it’s hitbox (specially EX version) has initial priority right in front of you, and not upwards (where your hurtbox is fully exposed).
So, if the opponent goes for a slow normal, a throw, or a kick, if he does not hit you for at the first frame chances are that you will reverse effectively with a wake-up ST. It could beat focuses too. That’s not bad for a reversal, despite of the lack of invincibilty (for example both Cammy’s and Ryu’s DPs aren’t armor break).
All of this makes me think it has been designed as a usable ground reversal, but willingly with big flaws against jump-ins. Just as the rest of our character. =)