I got a little lost when reading. I'm more of a fan of showing the code and then explaining it. And I prefer larger code examples because reviewing each line then showing the example at the end makes it difficult to follow. Also personally, I learn better with analogies. I know it's, imo, mostly about generics I just had a difficult time understanding in what cases I would use this article for my own app.
I'm pretty sure that most people reading will be unfamiliar/confused with extensions or Kotlin in general. It'd be helpful to see it geared more towards a dumber audience.
Nice read tho. I think if it was a little easier to understand it'd make a great article to reference