They are referring to current Syrian soldiers who fought against US backed rebels, ISIS and AQ at Aleppo and Deir ez-Zur. So they will be seasoned fighters.
Of course, they will quit the Syrian military to become mercenaries.
Nonetheless, the Syrian Gov will be paying their salaries. Its their payback to Russia who had helped them defeat foreign invaders in their country.
So now you have both sides recruiting foreign fighters.
Ukrainian fighters from unknown origin versus seasoned fighters from the Syrian war.
Personally, I suspect the Syrian fighters will have more experience in urban warfare - especially after Aleppo and Deir ez-Zur.
