Yes, and? 12 million subscribers =|= 10 million concurrent players (i.e. playing all at the same time).
Games grow, peek, and then contract. It's the life cycle of all games, so I'm not sure what this statement shows other than games aren't sitting on top of the world forever.
Which is kinda...duh. But Fortnite is still growing at the moment and doesn't look to have peeked yet.