That guy doesn't look like a cave or technical diver to me. ...
No, I am neither.
What is technical diving nowadays anyway?
I personally dive that equipment no deeper than 50m, rarely exceed 2h of dive time.
But I practiced with the equipment I own, so I know for certain that is way below the technical limit of the equipment.
Movements like that would just be silly in a cave, but those are for personally video feedback, so everything has to go fast.
The advantage in sidemount is the ability to freely position yourself in the water.
In nature cave animals often do not care much for up or down, sidemount enhances the ability to copy that for a human.
In open water environments that is less of an advantage, but it offers the ability to do things like what I do in the video: have fun testing some heavy doubles.
It is also a 'range extender' for most open water rec divers and offers several smaller advantages to tec divers.
With a good sidemount equipment you can do all of those dives:
normal evening dive, take one tank for a fast splash.
Weekend dive, take doubles or even add an extra stage.
100m wreck dive? Use every stage the boat has and drop all but two at the bottom for the penetration.
Cave dive? also no problem from an equipment standpoint, training may be a different matter.
Go ice diving in northern Europe or Canada on one weekend, then fly to the Bahamas or Mexico and use the same equipment for an extended cave exploration.
Afterwards you can visit family and dive with friends or the local club and again do not have to change anything.
I simply cannot imagine why anyone could miss all of the obvious advantages at once.