I hope that anyone on any platform will be able multiplayer together, otherwise it would be prudent for Larien to clearly declare as soon as able any limitations with a player base on different platforms.
The default for the overwhelming majority of games is that cross-play is not a feature. Gamers know and expect this, so no games declare it. You only ever hear about cross-play in the rare times when it is available, and then it is usually predominantly advertised.
