He's lying

"That was very good!" Jay said. "Now, watch carefully. See how fast I run."

He put himself in a sprinting position and started running full-out. Soon, he reached a speed faster than most humans had ever managed. Even more impressively, the machine kept moving just as smoothly, despite its heavy load.

"Amazing," the man said. "Just amazing."

"I thought you'd appreciate the novelty. But you shouldn't get too used to it. Once you return home, you may not be normal anymore."

"Why?"

"Because you are no longer human. You've undergone a metamorphosis into an immortal entity capable of enduring without sustenance or water. Not only that, but you have the ability to heal from any wound."

The man looked at him with admiration. "Then I can become immortal."

"Not quite. You don't actually become immortal, since immortality isn't possible. But if you want to remain alive forever, you can."