The simplest explanation put forward to date is that the spacing of the rapidly sent message was misheard or sloppily sent. In Morse code, determining accurate spacing between characters is vital to properly interpret the message; "STENDEC" uses exactly the same dot/dash sequence as "SCTI AR" (SCTI being the ICAO four-letter code for Los Cerrillos Airport in Santiago, AR being the Morse abbreviation for "over").Alternatively, the Morse spelling for "STENDEC" is one character off from instead spelling VALP, the call sign for the airport at Valparaiso, 110 kilometers north of Santiago.
As for the crash itself, wreckage was actually found in the late ‘90s. It was likely a textbook case of Controlled Flight into Terrain. “Plane hits mountain, everyone dies” was a common occurrence prior to the mid-1970s.