I honestly couldn’t agree more. Especially on sites like Medium which heavily focus on providing a modern and beautiful UI it bothers me a lot when I see stuff like this. They could so easily utilize the accent color that they already use for pretty much everything.
This is also not a pattern that could be too hard to understand for users. Knowing that holding the mouse button and moving across text selects the text is something that you can expect the average user to know. And a different background color for the selection won’t change that.