Whether or not you agree with Duffer's defense, he's right about one thing: "The Lost Sister" does feel like a pilot episode. In fact, more than anything, it resembles what is commonly called a "backdoor pilot," an episode of an ongoing television series which introduces a wide array of new characters, a new setting, and a new tonal approach in the hopes that it will be warmly received by audiences and lead to a spinoff. Recent examples of the backdoor pilot include "The Farm," an episode of The Office that zeroed in on the popular character Dwight Schrute's wacky family, and "Valley Girls," an attempt to examine the California childhood of Gossip Girl's Lily. Neither of those episodes became full-fledged series.
While the method has mostly fallen out of favor in the era of peak-TV -- spin-offs like Better Call Saul still happen but they're not often rolled into the pre-existing show -- "The Lost Sister" shares many stylistic qualities with old-fashioned backdoor pilots. (No such spin-off has been announced and the Duffers have not discussed one in recent interviews.) Like many backdoor pilots, it takes a beloved character (Eleven) and places them in an unfamiliar milieu (an underground gang of outcast street punks). It tells a self-contained story about Eleven's realization that Hawkins is her real home, but also leaves multiple loose ends and mysteries. For example, when is the sneering, mohawk dude Axe going to stop sneering all the time?