The skeptical me thinks that if Joshua and Wilder have already signed a contract and it is agreed to also be in Saudi Arabia then there might be some people that will make it quite certain that they will win their fights spectacularly with knockouts heheehehee.
Promoters have let that fight marinate for far too long. They don't have much of a choice but to fight each other if they want to remain relevant. If they drag it out further then either one of them could lose again and the interest in that matchup would be completely gone. If they are both fighting on the same card as main and co-main event then it is logical to conclude that there is an understanding for them to fight each other next. When there is so much money at stake for Matchroom and the sanctioning organizations it wouldn't be surprising if there was an early stoppage or a a controversial decision to ensure their desired outcome.