The USGS marker is located on a mound 300ft to the W-NW of the lookout tower. It looks lower than the base of the tower, but then why is the USGS marker there if it is not the actual highpoint? I wonder how many people miss that.
Your question raised my curiosity as well. Here are two possibilities. It may be the high point but, like you, I don't think so; we'll likely never know for sure. Alternatively, it may be roughly equal in elevation and if you saw the remains of some prior structure there next to it I'm guessing it was an antenna that was placed on the summit a while back to assist with other measurements. It may be that they chose a point that afforded better triangulation to other peaks as that's part of the purpose of those markers.