Agree, but only to a point. The safety has only one job to do on that play. He has to trust his team mates to do theirs. He didn't. He allowed Tua to keep him off his line by trying to do too much on the play. If he just does his job, that pass is not available. Of course, there were 2 other receivers and the RB wide open for shorter passes, so no matter what you were going to be okay, but the CB on that route had the underneath coverage. The safety knows this. The safety knows that if the receiver releases up the field, he is the only person who can defend him.
So, it boils down to one thing - that safety did not do his job. Call it a blown coverage, or call it something else, but that is where it starts and stops. If he does his job, the pass is never thrown. I understand the desire to give Tua all of the credit, but you can bet that Kirby saw it completely differently. And, as a DB coach, Saban probably sees it the same way Kirby does. Sometimes you straight up beat the other team, and sometimes they help. That safety helped.