@Zoidberg747, you misunderstand. I mean fighting against the computer on very hard, not training room. As for what you're reacting to, you're correct: against Shinnok, your default should be low, as that is his fastest option, but the moment you see movement indicating the overhead, you stand. His throw definitely complicates things, but luckily, if he uses it, you're out of the vortex. It's all doable, and is part of how I beat GGA Max last week at GBS.
That said, whether or not you can block the overhead on reaction is becoming a moot point, as top players are realizing that they don't need to use a reactable, and punishable, move to win. If you watch Mustard's recent run to victory, he almost never uses the overhead. REO uses it a bit more here, but it's still not much. If the overhead was as good as people believe it to be, that wouldn't be the case.