Which is a problem. The system as presented is actively worse than useless: it gives bad outcomes if you follow it. So either they are fixing it, which means you have to watch out for the changes, or they aren't, which means you have to continue to use 3PP or homebrew to make it work so why would you ever pay them for a new MM.
MotM is one of the worst products I have spent money on. It promised to solve problems it had no intention of solving, and gave next to nothing back for the cost of the book. If that is the template for the new MM, folks are better off saving themselves $60 and just using the 5E MM, or using that money to buy a 3PP MM that actually tries to solve the problem.
So which is it? Is WotC trying to trick you into buying the new MM, or are they lying to you about compatibility?