I wonder whether the author has considered taking the product to a paid level and what would be necessary for it.
Obviously, all contributors have some form of copyright, which may or may not have been waived depending on whether there was an ACL in place and jurisdiction. So he would need to get permission from the copyright holders, maybe in exchange for a percentage of the profit.
ANd that gets rather looked on here as the authors being deceitful and not really Open Source doing a bait and switch.
Changing the license of already existing code? You might not be able to do that without permission from other contributors, I agree.
But it's MIT license. We can open a company tomorrow, take that code, and start selling it. Further development and improvements of the code could be trivially done openly or behind closed doors. FWIW the author themselves could do that if they wanted.