A lot of people were dubious about Sin being Lancelot, after Gowther saw Sin and didn't mention the fact that he's one of the 4 horsemen of the apocalypse, but that's no scoop and I have the explanation. Because Gowther just doesn't know, remember, at the beginning of the story none of the 4 horsemen of the apocalypse looked like, Ironside kills Vargnese for the possibility that he is one of them, he doesn't know what they are, later it's revealed by one prophet that one of the 4 knights has greenish hair and wings, then Ironside is enraged because he knows Percival is him. That's the point, they don't know what the 4 knights of the apocalypse are like, only Percival, and it's something recent, the prophecies must be updated as time goes on, and Arthur's knights and others will eventually know what the 4 knights of the apocalypse are like. apocalypse, but currently they don't know! The image of the 4 knights of the apocalypse is something shown to us readers, the characters are not aware of it, if Arthur's knights don't know what the 4 knights are like, chances are Gowther doesn't know either, so if Sin is lancelot and Lancelot is one of the 4 horsemen of the gowther apocalypse just doesn't know it. Lancelot may be disguised as Sin not because he is one of the 4 knights of the apocalypse, but because he is the child of one of the 7 deadly sins, threat enough to be hunted.