I've never heard of that. The heat comes from capsaicin that is found mostly in the seeds and the ribs inside the pepper. If you remove the seeds and ribs (with as little smashing, breaking, tearing that will release the capsaicin and spread it on the rest of the pepper) then you'll get rid of most of the heat.
Since capsaicin is not really water soluble, I don't know how much soaking peppers in sprite would do... other than adding a sweet component to the peppers. It is fat and alcohol soluble so perhaps a soak in some oil or alcohol would help (which will pick up the heat and might be good to use for other applications).
-Dough; 22.5" WSM, Traeger Lil Tex