Seems like that is essentially what he was saying, he's letting guys shoot if it is in spots that they can shoot from and later on he will address what is an open shot vs contested shot. So he will probably reign Mullen in with that part of his shot selection, but not the spots.
Let's be honest though, we'd rather see Mullens shooting three's than long two's that he hits at a lower percentage and mid-range two's that he doesn't hit that well. If he's popping to 20 feet instead of three, he's a worse player. Of course the one we'd like to see more of is him rolling to the basket. Last year only 18% of his shots were at the rim, he needs to get that back up to his 30% at the rim in 11-12, then convert the majority of his long two's into three point shots, and his value as a role playing stretch big will start to climb up.
I know, easier said than done, but I think he's capable, hopefully.
Last season eFG%:
10-16 - 35.4%
16-23 - 31.5%
Even if we're just doing regular percentages, he shot 31.7% from 3PT. There's probably no reason in his mind for him to aim to shoot the mid-range jumpshot. He's not a primary scorer for the team, and if he can't get shots at his best spots (at the rim or 3PT), then he should be passing to a better player really. That's how you become an effective role player.
To the other point about the bench being shut down, you would actually think the bench has a bit of an advantage because while many teams can guard one perimeter big, not a lot of teams have two big men comfortable with guarding the three point line. So a lineup with Mullens and Jamison should open up the lane for Collison, Crawford and let's not forget that Barnes will be back.
Theoretically, both the opposing bigs are now hanging out on the perimeter, that's gold for anyone who wants to slash, cut, or get to the basket.
We have to also take into account that on the bench, Crawford is the #1 option, Collison #2, and then the other guys are all third options, so the versatility of a guy like Mullen's offense isn't really that important as long as he plays the role demanded of him well.
Mullen's isn't so successful with a lot of the in between game, so him complimenting Crawford and Collison as opposed to trying to make his game any sort of focus is more important. In an average game the bench is only going to be in together as a whole unit for at most the equivalent of a quarter's worth of minutes. After that, any bench player that get's more is playing with starters and they become even more specialized in that setting.
...and like Voyeur said, in close games, and in the playoffs, this isn't likely an issue as one of Blake and/or DJ will be one the floor for most of the game anyways.