the guy who killed him is also a mercenary. and poles live in ukraine, there are lawful combatants from the polish citizens of ukraine.
e: the article says “volunteer” but that still seems like something that would need a trial to determine whether they lacked protections before a summary execution. that should apply to any participants from neighboring countries’ ethnicities
you could also say the folding of the ukrainian foreign legion into the territorial defense forces formally makes them not mercenaries with an equal legal footing. either we stringently follow the geneva conventions and there are no mercenaries on either side, or we call a spade a spade