The USGS topo map shows 2 contours of 1420 feet (20' interval), the southern one with a 1422 spot elevation. The King County iMAP GIS site shows 2 contours of 1430' (10' interval). The southern area is on private property, and the King County map show that it is by far the smaller contour and it almost entirely coincides with a house. The northern contour is on public state forest land and is larger and is also undisturbed natural ground. This northern contour almost certainly seems like the actual high point of Grand Ridge.
A good unmaintained trail on public property leads from the Grand Ridge trail to within 0.3 miles of the high point. The last bit is some nasty bushwhacking to the indicated coordinates, an obvious rise in the forest well away from the nearby private residences.