30 seconds in and I’m checking out. I can already tell it a propaganda piece intended to make me angry. Apparently they think the use of mercenaries in war is new or has somehow exploded in use. Mercs are maybe the 2nd or 3rd oldest profession. Not interested.
To figure that out you'd first have to decide how you want to define "mercenary". Depending on how you define the word, either almost everyone was a mercenary or, if you use the current Geneva Convention definition, almost no one was.
The truth, as usual, is probably somewhere in that murky, uncomfortable middle ground that no one likes to look at or acknowledge.