He absolutely completed the process of making the catch. There is no description of the process other than going to the ground. When his first step touched the ground, he completed the process. The NFL rulebook does NOT and NEVER did define any other description of the process. There is no expanded version of what the process is, so Dez had the catch as soon as two feet were in bounds with possession. When the ball had been caught through seven distinct football moves, it then popped out. It was a catch, SEVEN times over.
BY WRITTEN RULE... CATCH.