"Tucking the ball" as a football move has nothing to do with how close the ball is to the body. Receivers in the open field rarely hold the ball against their body like a running back does when hitting the line of scrimmage. This is not about fundamentals of ball protection in traffic, it's about some act that is performed to show that the ball has already been caught.
No receiver willingly takes one hand off a ball he hasn't caught yet.
Why did the commissioner appoint the catch committee to clarify what constitutes a catch in 2016?
Why did the catch committee specifically add "tucking the ball" and "additional steps" as football moves?
Why is Pereira now saying Dez's catch should have stood?
It's because he can't continue to defend Blandino in the light of the reversal of the James play, which he knows was wrong. It's because the 3rd part of the catch process, the time element, is too subjective to judge without looking for the football move. If you just go by "upright long enough," there is no clear diving line between receiver and runner. Blandino's addition to the rule book is flawed and should be removed, just as he himself was removed. Unless you believe he wanted to spend more time with his family.