I'm flabbergasted. I plugged Hornday's data for a 24" barrel the JBM Ballistics calculator using the environmentals described the video. The result says the bullet goes subsonic at around 1,400 yards. Link below.
2,000 Yard Shot with 6mm Arc and New Harvester EVO - YouTube
2,000 Yard Shot with 6mm Arc and New Harvester EVO - YouTube