No, it' because whoever made the site is ignoring the age-old simple practice of showing something (even the text "This website requires javascript") even if not all resources have loaded.
It is entirely unlike the examples you gave. Especially because his browser does show ~99% of the internet properly (with various degrees of functionality, but still something does come up), and that there spiders, proxies, screenreaders, and other useful software which will also see nothing on this page.
All major screenreaders work from the DOM and the accessibility tree nowadays, and have done for a long time at this point. Dynamic content is no problem in and of itself.
I did a lot of tests on this and Google just didn't wait, no matter the configuration. I tested using webmaster tools. The App was built using React, and I ended up implementing SSR to fix the problem. There are a lot of posts that hack this embedding JSON directly on the page. I didn't find any post that figured out a way to make this works through async API requests. I would thank you if you point one in that direction. Maybe I'm doing something wrong.
Interesting, never occurred to me that react-router could be the problem, I will investigate that. But this other post [1] didn't use React at all and made API requests to GitHub with the same problem.
It is entirely unlike the examples you gave. Especially because his browser does show ~99% of the internet properly (with various degrees of functionality, but still something does come up), and that there spiders, proxies, screenreaders, and other useful software which will also see nothing on this page.