I don't see how you can model a chain stay as a cantilever beam.
Because it is one. It does suffer some compression/tension loads but those cause no measurable deflection.
The seat tube, down tube, and seat stays all resist a force like that, and I think the rear hub also plays a significant role.
The seatstays are so thin and so long that they provide no stiffness to the rear triangle. (except for being incompressible, that is not changing their length).
And in use, the real stiffness of the frame to a side force is dominated by the headset.
Either I misunderstood this statement or this is horribly wrong.