Just a guess, based on the fact that even in Steam games, Valve's servers are only used in Valve's games, while in games from other developers, servers from those developers are used. Steam is simply a store and a communication service. Of the things that can be called a server, there is only Steam Cloud, which provides synchronization of saved games, but it has no direct relation to the game servers on which players play. In any case, Ubisoft is in charge of everything, so it all depends on them.