Sean -
I think that's right. MacK often didn't practice what he preached. But Bobby Jones was a unique boss who understood golf architecture better than most and gave MacK more freedom than he usually got.
Might it be that the effect of the Great Depression was less about changing MacK's thinking about bunkers than it was about changing the thinking of the people that hired him? That is, until the economy turned, everyone wanted lots of pretty bunkers, so MacK felt obligated to build them. Then the GD hits and the expense of bunkers suddenly becomes an issue, at least for the few courses built after 1930.
Bobby Jones, however, had fairly sophisticated tastes in gca and didn't need the GD to see the virtues of what MacK wanted to do. Just thinking out loud.
Bob