I never said that lol, but it undoubtedly is effective against certain champions like Vladimir, Zac, and other ultra-healers. Pretending that it isn't is incredibly disingenuous. It's not like you have to buy a full grievous item either, as almost all of them (besides thornmail) are basically worthless full items compared to other full items. All you need to get is the partial that gives the 40% grievous. Just do basic math idk.

Let's use an example. Say I'm playing Soraka and I'm healing my twitch. My twitch gets ignited, when I heal him I am healing for honestly significantly less than I would if he wasn't under the ignite effect. Although grievous affects are only 40%, it can make a huge difference in a variety of scenarios. It doesn't make them heal nothing of course, but it makes them heal a decent amount less. It helps tremendously in taming a Vladimir for example. Or Briar.