Cross platform matchmaking is enabled by default in Skull and Bones. If you would like to turn this feature off, then you can check out this Seeking Tech guide to see how you can properly do so in this online title.
1. If you haven’t already, exit the game and head back out to the Main Menu.
2. From there, go to Options and then select Settings.
3. In Settings, make sure you are viewing the Gameplay tab.
4. Next, scroll down to Crossplay. You should see the following description:
Enable this setting to be able to play with players on other platforms.
5. Make sure that the Crossplay setting is set to Off and then exit the Settings menu.
You should now only be matched up with other players from the same platform.