My understanding of this is you turn the legs green (swollen) and the rungs very dry (shrunken) so that wen you drill the hole for the tenon, the tenon swells and the hole/mortise shrinks, it becomes very tight. And who ever assembled the chair (bodger?) even paid attention to the grain orientation to enhance the fit.
I'm guessing you already knew that.
My friend who made chairs had a box of sand with a heater under it and he inserted the tenons into it to shrink them as much as possible and then inserted them. I think he put them back on the lathe after they were fully dried and still hot and re cut the tenon with a wrench he had sharpened for the purpose of cutting a very exact tenon. His chairs held up well.