Here's my personal interpretation.
Dez doesn't establish himself in bounds until his left elbow hits the ground. If he cleanly caught the ball initially, then the reach would be considered a football move.
However, he only established one foot down after controlling the ball, so he's still technically in the process of the catch until the elbow comes down, at which point he needs to control the ball throughout the process to the end of the play for it to be ruled a catch.
Once his elbow is down, he's down by contact if he holds onto the ball as it's an elbow, and he's officially in bounds. But he still needs to hold onto the ball to the end, or it isn't a catch.
Think of it in CFB terms without contact. You can't make a diving catch, have the elbow hit to be down/in bounds with possession, and then lose the ball as the rest of your body comes down. That's obviously not a catch. I view this in the same way.