I think it makes a ton of sense. Steam targeted Ubuntu from the beginning.
But even if they are working with Ubuntu for some of the reasons you mentioned, that doesn't mean that they have to go with Mir. You could install Mir for any distribution. You could install Wayland on any distribution. You could just keep going with X because at least it's stable and people know about it.
But even if they are working with Ubuntu for some of the reasons you mentioned, that doesn't mean that they have to go with Mir. You could install Mir for any distribution. You could install Wayland on any distribution. You could just keep going with X because at least it's stable and people know about it.