Some of the points aren't that convincing (Kenobi was a padawan when he defeated Maul) but some others have merit. The Stranger's identity was revealed too easily, which suggests there are more twists and turns to come. They don't claim to be Sith and may be revealed to be some kind of cultist or unaligned Dark Side Force user, some suggest an early Knight of Ten but they've already thrown in the witches, so it may be some other new group.
However, we don't really know who the acolyte of the title is - it's the stage before becoming an apprentice, so it could be the Stranger is the acolyte to someone like Darth Plagueis. However, that might cause complications unless there's some scheme in place to make it look like he is an unaligned force user.