Yeah, the saltpeter (potassium nitrate) is really the key ingredient here. It’s the rapid oxidizer compound that gives your anti-Gorn cannon its boom boom. Without that, mixing all the other stuff together is just going to be smelly and disappointing.
Did the mushroom learn to control a robot, or did the scientists figure out how to connect a robot to a mushroom in such a way as to make the regular processes happening inside the mushroom trigger a set of robot legs? Because the article makes it seem like the mushroom is intelligent and has agency, and was thus far only lacking the proper robot body in order to express that; but the video makes it look like the legs were all pumping in unison, and the resulting movement was more or less coincidental.