Would you extend that argument to tabbed interfaces as well? Why should browsers support tabs (and an inconsistent interface by each vendor), when you can just open a new window instead?

The tabs reuse resources of the browser, and the browser does it really well - I think it's not even arguable that browsers are more complex than the OS GUI API, this is why e.g. Windows 11 uses react.js in start menu.

So if you create a webpage that is so damn advanced that it beats the browsers OR it somehow reuses heavy resources within one webpage, I'd say this is a good justification. And IMO the OP link isn't an example of that.

One could argue that this affordance should be provided by the OS for a unified experience.

Even better, WMs can (and sometimes do) support tabs!