Article seems to talk about the "flat chat-like" view (called conversation threading) rather threading in general. Threading as seen in mutt theoretically exists since email was standardized in RFC 822 (predating even Usenet) due to Message-ID/In-Reply-To/References fields in header (which mutt uses).
Yes, the article seems to be confused in a few ways. The fact is that Gmail is not a real email user agent, because of a handful of deficiencies; the most critical of these is the lack of conversation threading. This article (https://lwn.net/Articles/837960/) shows in detail how Gmail is broken and how a non-broken email client (mutt in this case) does things more usefully.
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