I fish strip mine lakes fairly frequently, but of all the different ones I have fished I can only think of one that has a flat like you described, and one similar to it.
The one that fits that depth more accurately had a good amount of vegetation on most of the flat. It has been a while since I have fished that lake but from what I recall there were good numbers of fish positioned throughout the flat around this time of year, particularly in areas where there were variations in cover (density and type) and near channels from deep water into the flat.
The other flat is slightly deeper, 5-7 feet, and doesn't sound like it had as drastic of a drop-off. This flat has sparse vegetation and I typically focus on lay downs and on the areas where there is vegetation, but less on the edge itself.
It sounds like the recommendation that was given may have been to fish the shallower water or right against it. If your there either early or late in the day (less wind to thrash you around ) I would try a buzzbait if the cover isn't overly heavy or a hollow body frog in the thicker cover or anything that covers water quickly. If you choose to slow down your presentation focus on areas that are slightly different than others around it. If you find fish off the edge of the flat try running a spinnerbait parallel to the drop-off particularly if its windy.
Hope this helps
