I dunno if this has been found in the mean time, but I've been able to reliably reproduce this glitch. Simply run to the location in the middle of the oily marsh in the middle of the map shown here:

I'm not even a speedrunner myself, but I've been watching runs and recalled this glitch and was amazed when it happened to me. Since that location looks pretty close to the center of the mission area, my suspicion is a scripting or map editor bug also allows the cutscene to trigger from that location.