If there's a rematch clause it's likely to be the other way only. But equally Wardley realistically wouldn't win without a KO - and would Usyk want to rematch with someone that knocked him out, or would he just call it a day? I don't think rematches for that one, if it happens, are a major issue.
Usyk can do what he wants, do you really think he is stupid enough to allow Wardley a rematch clause if Usyk schools him? Even Fury 3 would probably be better than having a rematch against Wardley after absolutely schooling him.