If you listen what McClay says in that clip of the war room, he says that “the guard” (Bergeron) would be depth while the DT would be a starter.
Here is what McClay says in that war room clip. This is word for word what McClay says:
Will McClay: “Personally, I would go with Mazi because he helps us now. With the guard, I love the guard too… my question is, you’re adding something to it where we have depth. I think the defensive lineman could be a starter AND something for the future.”
McClay is saying essentially that Bergeron would be for the future whereas Mazi is a starter in the present as well as the future.
They just really liked Bergeron and considered him as a BPA “tie” with Mazi. They weren’t getting him to be a starter this year. McClay broke that tie by saying they could use Mazi this year. McClay also says that they’ve had highly rated OL available at their pick pretty often but that it had been a long time since they’ve had a DT rated as highly as Mazi.
It made Mazi the pick. They didn’t expect “the guard” to start this year.