In this panel we obviously see Geto's body reacting to Gojo's words. It's not as if the body had some kind of muscle memory that reacted once it heard Gojo by like freezing or twitching. It was a very obvious action of Suguru trying to choke Kenjaku/himself to be free and stop being used (like Satoru was saying).
So once that you discard the idea of muscle memory, we hope that Suguru is actually alive inside of his body just not in control. But how? He was supposed to be dead but we don't actually see the moment that Gojo finishes him off or that he finally dies, though by Satoru's reaction at the station we know he believed the man is dead. Then there's two options:
- Somehow Kenjaku was there when Geto was claimed dead and got in his body then. And Geto was still alive despite the looks so he was trapped with the curse.
- When Geto's body was discarded (because you know he was in school ground I think) Kenjaku found it and decided he was a good and strong vessel.
In both options Suguru's life would depend on the curse. As we know, his technique is curse manipulation by ingesting them but we don't if he could do the same with curse that inhabited his body. If so, which is what I think, the energy that Kenjaku holds brought him back to life or it's been giving him stenght for over a year.
That covers the theory on how can he be alive plus gives us the hope that he will be able to take back his body if he keeps on feeding from the energy. Because this panel reminds me of this other one of Itadori stopping Sukuna earlier in the manga when he had only eaten one finger. That means that Kenjaku isn't as strong as he could be, giving Suguru the chance of moving his arm.
I imagine that inside of the body there might be a domain (much like Sukuna's in Itadori's body) where they could go against each other and Geto would be able to exorcise the curse.
Just a theory.
