It looks like the last survey was 4.5 years ago. It’s a shallow lake so things could have changed a bunch in that time.
I personally wouldn’t say 4.0 per net is anything to get too excited about, although it’s ok for a metro lake. My frame of reference is Waconia which just surveyed at 5.6 per net this past summer and I don’t consider it to be a good walleye lake. There are lakes a little farther west with far more walleye per gill net.
McMahon actually looks like a much better crappie, bluegill, pike and perch lake but a lot of things can change in 4.5 years. It might have gotten fished hard the year or two after that last survey.
I’d look for bodies with more recent surveys, especially in the metro area where there is a lot of pressure chasing hot bites.