I assume he was referring to the increased thrust that was probably obtained
with the turbine installation, which would create higher forces on the
structure transmitting that thrust to the airframe.

I assumed the same. The round Pratts were 550hp engines, and the STC'd
PT6A-27 engines are flat-rated to 650hp. While the increased thrust
might add stress, my assumption was the weight reduction of the
turbines and their much smoother operation might nullify the power
increase as it relates to airframe stresses. It seemed a safe
assumption that that's what he was suggesting.