A functional interface needs to be understood. With each new interface you use, you load a new set of metaphors and interactions into your brain to help you navigate, understand, and use whatever it is that you are using. The interface needs to be relatable, or at least offer an explanation of how to use it. Having more experience with interfaces will make you more familiar with different design patterns and such, but there is a limit to how much we can relate to.
This isn't about readability, but rather usability, so I find your examples poor. It's not just about typography and whitespace. Yes, they are simple and on the "same visual plane", but they follow design patterns that have existed on the web for many years: Navigation placement is on the top and left, links are either underlined or highlighted. On the other hand, svbtle is using bizarre navigation visual cues at the top which need some level of inspection or trust to use, and medium doesn't have a standard navigation at all. They are designing away elements of the webpage that people are familiar with.
This isn't about readability, but rather usability, so I find your examples poor. It's not just about typography and whitespace. Yes, they are simple and on the "same visual plane", but they follow design patterns that have existed on the web for many years: Navigation placement is on the top and left, links are either underlined or highlighted. On the other hand, svbtle is using bizarre navigation visual cues at the top which need some level of inspection or trust to use, and medium doesn't have a standard navigation at all. They are designing away elements of the webpage that people are familiar with.