Nop wrong. details/summary tags are not supported on IE and Edge. You have to use a polyfill, hence JS.
I think he's more-so implying it fails gracefully, still delivery an adequate content-complete experience to the end-user. If you use details/summary in IE and Edge, the user is still provided all of the content placed in the marked up, the interaction of close/open state is just quietly omitted.
I think he means the content is still accessible. It won't function the same but it's still usable.
Worth remembering IE11 is the only version of IE still in support and isn't that widely used anymore, it's not that unheard of to say you don't support IE.
Also, Edge will soon move over to the blink engine which will give it the same support as chrome.
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