that vibranium body is indestructible and the mind stone is literally in his mind so he probably telepathically already knew what thanos was about to do. A killing machine like that knowing your next move is scary as fukk but they never displayed what the mind stone can really do in the mcu
Ultron
was the Mind Stone at that point.
When Thanos stepped up, all of the other stones vibed with Ultron off the bat.
Thanos could not process anything but wanting the Mind Stone.
The Mind Stone (Ultron) knew what had to be done immediately.
Ultron had a moment of curiosity followed by Thanos getting washed in a sec.
AI Infinity vs Biological Infinity...