Actually, the people that bought the game on the Larian Vault got Steam keys to go with it, so it's only the people from GOG who don't have any way to get Steamworks matchmaking.
I figure that the decision to rely only on Steamworks matchmaking was to prevent having two branches of code to deal with when it came time to optimize or fix bugs.
Edit: By the way, they did clarify what they meant in the exact post you quoted.
Steamworks is being used for online lobbies, leadership boards and inviting friends. These features are not available if you don't have the game installed on Steam. What you can do with non-Steam versions is play directly with a friend over LAN or online if you know each others IP.
