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.
They are still mercenaries no matter how you want to try and twist it.