That’s like saying “It’s not sex with friends so it’s not cheating”.
They’re mercenaries despite the mental gymnastics.
No, it is like saying a city policeman is not a vigilante.
Mercenaries are real and Russia uses them, being a foreign volunteer or even a native volunteer doesn’t make you a mercenary, not being a member of the nation’s official military does that.
The French Foreign Legion uses mostly foreign volunteers but they are part of the French Army, they are not mercenaries.