Why? God could have taken him out of his body at anytime He wished to..dead or not.
Indeed. But the whole premise of this "vision" is based on a faulty claim that the man was "clinically dead" for 21 minutes. No, he wasn't. They don't stop the circulation in the body to do heart surgery. They bypass the heart and have the circulation conducted by a machine. Therefore, his brain was still getting oxygen. Therefore, he was not clinically dead.
It is very possible this man is telling what he thinks happened. What he fails to realize is that people under general anesthesia often have very weird "dreams".
No. . .if God were going to take this guy to heaven and show him purgatory, He would do so like He did with John, not while he was under general anesthesia.