> The phasing out of third-party cookies has nothing to do with the "remember this device" functionality, because those are almost always powered by first-party cookies, which are not being phased out.

Cookies are cookies, not a "device". You could theoretically copy whatever cookies they put in to authenticate in other place and get automatically logged in.

> The author has configured their browser to throw away cookies when the last tab closes

If web was supposed to be stateful, this would be a bug, and the author would have had to dig through experimental or advanced settings to enable it. But because web(HTTP) is fundamentally stateless, it is a feature, and it's a feature that every browser exposes to normal user. Your browser is not expected to keep your cookies, just like your browser isn't expected to support javascript to access a website, or that you should enable them even if you could.