Fading in is justifiable when you're adding new content on top of existing content, and need to draw attention to the fact that something changed in that part of the screen. None of that applies in the case of fade-in during scrolling. The user is already scrolling, everything on screen is moving, and new content is already expected to be coming in to view at the bottom of the window. Adding animations on top of all of that doesn't help anything, and just distracts from and delays presentation of the content the user was already trying to reveal.
It's just a way to draw attention to text without relying on typeface modifiers.
The real designs encourage users to notice what they're scrolling past as it flies by on the screen. That temporary urgency is why it's an animation. This is something that weight, height, underline, etc. cannot do as clearly. If you notice it's usually on the h-tags only, not every line. That also communicates the page structure. It's clever and attractive when done right. It's not a new idea, and nobody was complaining about it until now.
But that raises the question...