And the reason he regained possession OOB is because he didn't survive the ground. His knee hit in bounds. If he had survived the ground at that point it would have been a TD. But because he didn't, and didn't regain possession until OOB in the EZ, it was a touchback.
Likewise, the defender had a valid fumble recovery because Lewis lost the ball, creating a loose ball, was trying to re-control it but didn't survive the ground (because the defender stole it from him before he could get the necessary control over it), therefore the ball was still considered a loose ball, therefore Lewis was not down by contact, therefore a fumble recovery.