The Morgan Creek work may look like subtle grading, the the gradient on the photo appears to be quite a bit. My guess is that the differential is at least in the 3% range ... 3 feet of fall in just 100 feet.
At Peacock that would put is in saltwater muck! When you dig an irrigation trench at Peacock that is just 3 feet deep the trench will fill with water in about 15 minutes.
Importation of material would have caused a host of issues: Re-calculation of the entire floodplain, possibly triggering a FEMA review of the official U.S. Floodplain maps. Typically a FEMA flood plain change takes 3-4 years...and that is only if the Federal Govt. is willing to listen. The Northern Calif. area is expecially sensitive to changes in flood plain mapping. It rarely gets done quickly.
So, besides the physical challenge, you have an approval challenge. And...perhaps most appropriate to consider: The site was a rather flat marshland in its ancient configuration. In fact, it was dead-flat except for the two hills — one for the clubhouse and the hill at No. 2, 3 and 6. Wm. F. Bell use the hills well in his routing. We did have a few routing options that involved the hill out on the course (Nos. 2, 3 and 6) better, but it would have meant re-routing 5 holes.
A reprieve for a hilly site is often softer, less dramatic greens — our approach here was to turn that notion upside-down; to create some dramatic greens as a contrast to the flat-ish site. In my option, the bunkers, greens and even the trees are all made more intensive because the "meadow" is flat and level.