This limitation is very much open to interpretation. Consider health as a resource that can be used in more that just the one standard way. This doesn't just have to be Hit Points, you can get creative with it, anything that can pass as health will work. Physical and mental health are both valid starting points. On top of this, the word 'tool' has a few different definitions itself, more than just "a device used to perform or facilitate manual or mechanical work". Also note that this is not specifically the health of the player character, but health in general!