Rethinking that. The thread you reference discusses the compatibility of the PT911 and PT940, whose Frames are the same and the Caliber of their Slide/Barrel Assemblies different.
The PT909 frame on the other hand is adapted directly from the PT92. Now my PT908 is a direct ancestor of the PT911/915 Pistol and I did try to see how the PT908 Slide matched up with my PT92's Frame.
Although there was some match up, it was very very tight. So much so, I decided not to force it on. The PT909 Slide might fit a PT911 Frame, depending upon whether Taurus fitted the PT908/911/915 Slide to the PT92 Frame or fitted the PT92 Frame to the PT908/911/915 Slide. In one case, fit would be a bit sloppy, the other, too tight.
There is also the possibility that Taurus changed the PT908 Slide to being just a slightly wider Slide to accommodate the higher capacity PT911/915/940 Magazines, making that Slide a PT911/915/940 Slide only, and those dimensions might be compatible with a PT92 Frame. That would make the PT909 Slide directly compatible with a PT911/915/940 frame. As the double stack magazines taper to a single feed point, it would not have been really necessary to widen the Slide, but there is no way of knowing exactly what the Taurus design team did. Just looking at them, the PT911/915/940 Slide does look more directly compatible to the PT909's Slide. Given Taurus's penchant for lack of Magazine compatibility between models, it's not something I would want to bet the House on!
Blued Finish Taurus PT908, 8 shot, 9mm compact