That's a common myth that WD-40 isn't a lubricant. It is true that it is mostly a solvent. If you WD-40 something with grease on it it will probably have less grease than was there before. However WD-40 does loosen up dried up grease, and it does leave an appreciable amount of oil that acts as a lubricant, just usually much less than what is needed for most bike parts. Most of spraying STIs with WD-40 is to loosen up dry grease, not displace all of it. Even if you did, it still acts like a light lubricant, and because of the low viscosity due to the high solvent content, it will work its way into small gaps and leave some light oil which is often enough for shifters.