This is false, recently the details element has gotten support for grouping them: the [name] attribute. This effectively enforces tab-like semantics where only one of the grouped details elements can be open at a time.
This is a quite recent addition and the modern web is evolving too fast so I wouldn't put it past myself for missing this :)
Yay for progress and for JavaScript free solutions!
No, it's still true. I'm aware of that hack, but unfortunately it doesn't solve the problems with pure HTML and CSS tabs.
Crucially, the `name` attribute does not semantically turn a group of <details> elements into a set of tabs. All it does is introduce the (visual) behavior where opening one <details> element closes the others.
I posted a list of accessibility issues with the <details> hack in another comment: https://news.ycombinator.com/item?id=46415271