In Chapter 9, Vertical Tail Design and Spiral Stability, Lennon's logic is almost as hard to translate into standard mechanics as Grant's.
Perhaps a clearer explanation is that since the craft is turning in yaw, the center of aerodynamic side force moves forward, toward the end that is moving up wind. It moves, more or less, along the line connecting the front and rear centers of lateral area, so it ends up above the cg and and rolls the airplane away from the direction of the turn so that the wings' lift opposes the turn. This is not complete, even at this level of approximation, but I think it is at least closer to standard mechanical analysis.
It is not clear to me how much analysis and how much experience lead to this successful "theory".
It appears that to really understand this practical advice, one still needs to read a regular aerodynamics text book, as I did a long time back. Probably why things are explained (or rationalized) so strangely is that Charles Grand was largely self taught, and Lennon learned directly or indirectly from Grant. I suspect that a clearer analysis was already available, at least, by 1941 when Grant's book is dated, but no one seems to have unified the model and piloted aircraft technologies.
I do not accept Lennon's idea that separating the front and rear masses is important to simple spiral stability. Angular momentum is clearly not important for a wide spiral of a small model, and I don't remember it being considered, at this level of detail, in the text book (for craft with much more angular inertia). I think it is customary and correct, at this level of detail, to treat the craft as a point mass.