Check out this discussion in the comments section regarding 18% gray cards and 50% gray (or “middle gray”) in an otherwise straightforward advice post. The confusion is around using what appear to be the same units of measurement (“% gray”) interchangeably, when they are measuring different things.
In this context, “50% gray” is referring to a perceptual measurement. That is, it appears to the eye to be halfway between white and black, colloquially referred to “middle gray”. An “18% gray” card, used to set this “middle gray” perceived value in Lightroom, is 18% reflective; it reflects 18% of light that hits it, producing what appears to the eye to be middle gray.