He did not leave the plane, he said he was on the previous flight from Delhi to Ahmedabad, before the plane then went on to do the Ahmedabad - London flight when it crashed. You can see his flight ticket in the tweet.

Ok, thank you, didn't realize. He wrote two hours and I expected this to be another long-haul flight.

It reads to me as if he was on the flight before the doomed flight.

India is a large country, so a plane travelling a route like Delhi→Ahmedabad→London isn't unusual, with passengers able to board and disembark in Ahmedabad.

(There may also be security rules like requiring continuing passengers to disembark with their hand luggage before reboarding. I don't know, it's 15+ years since I took a flight like this.)