Officially, the benefit is only for SkyTeam-operated flights:
1 extra checked baggage item on flights operated by Air France, KLM, Aircalin, Air Corsica, China Southern Airlines and SkyTeam partners
So when checking in with a non-SkyTeam airline, there is sometimes/usually no extra allowance. But my last time with Air Baltic (KLM light fare), they didn’t make me pay to check my bag.