it is mentioned in the article indeed; it's an awful solution that is poor in performance and break the accessibility

Thanks. I was reading too fast and missed the iframe reference in the article

Why would the performance be any better with another tag?

A frame is a separate rendering context—it's (almost) as heavyweight as a new tab. The author wants to insert content from another file directly into the existing DOM, merging the two documents completely.

Negligible twenty years ago. But yes, if there's an improvement it should be merged automatically into the same document.