The signal may have been for simultaneous engagement on radio command; or engage target 2 and 3 on the report of the shot for target 1. Might have been engage first target at first opportunity and then terminate additional threats immediately. There are too may 'might haves' to say with any certainty what happened, and the folks who know aren't saying for obvious reasons- op sec. What is not speculation is that our military has fine young folks in all branches, with SPECWAR folks who train for any and all engagements. God bless them all. We can be thankful they are willing to accept these missions and that they have the skill, bravery and determination to emerge victorious.
Night vision optical sights and/or momentary illumination would solve the 'darkness' problem and the 'rolling seas' matter has been addressed.
You can still do pretty good 'night/moving target work' with quality conventional optics and good training. When I was involved in LE sniper stuff, we hung a 2 liter bottle in a shoot house, suspended from the ceiling on a 3 foot cord. A mag light in the corner was the only illumination available to the designated marksman, set up on a pitch black hillside, 100 yards away. On radio command a brave soul in the shoot house would give the bottle a hard swing and then run like hell over the side berm. On his clearance, the DM would engage the bottle as it swung, through a 2x2 foot 'window'. A 'hostage' dummy (we could never get anyone to volunteer for that job- dang sissies) sat about a foot off from where the bottle would be if it weren't swinging. Our better shooters got to the point they could routinely bust the bottle as it swung- and one shot was all you got.
Never underestimate the effectiveness of a well trained and committed individual, with a good rifle in his hands. Folks fitting that description are precisely the reason we are free.