This is my previous life. Based on the video, and using the old periscope ranging reticle formula, this is about a 3500-5000 yard shot. Point blank for a MK-48 ADCAP.
Classic 90-degree kill shot. Odds are that the Iranian Corvette/Frigate had no idea the sub was there. I'd put money on that part.
Looks like they went for an "acoustic" shot, as opposed to having the torpedo home on "center mass". So the torpedo homed in on the screws (propellers). The torpedo is set to detonate underneath the hull, and use (a) the warhead blast and (b) the resulting dynamic hydraulic shock to disable the target. Blew the stern off.
That tends to slow them down.
One thing I can tell you for sure: when the name of the sub comes out, and it eventually will, those guys will never pay for another beer at the local VFW.
Run Silent, Run Deep. Death From Below.