Yes. In Nicholson's day "panels for framing" tended to be around 1/4 inch thick. Raised panels had sort of become a thing of the past. It is relatively easy to make drawer sides from 1/4 inch material without a shooting board, but making a good joint in thin stuff like this is hard because a slight difference from one side of the edge to the other will change the angle dramatically over a panel width of 16 inches or whatever. In the case of a harpsichord soundboard, you need quarter sawn material several feet wide and 1/8 (or less) thick. You can imagine he difficulty of jointing (and clamping) this material.
The routine use of a shooting board on end grain, to insure that the surface is square to the edge and square to the face of the board, is a relatively new idea. The shooting board used to be reserved for pieces that were excessively thin or short joints.