Here is how it works https://jmperezperez.com/medium-image-progressive-loading-placeholder/

I don't think you're late to the party! I first saw that like a few days ago I think it's kinda cool and going in the same way that they already do with placeholders for unloaded content !

I might be a bit late to the party, but the progressive loading is neat! This is what I'm talking about: gif