Its actually right in the posted link:

The “Sold by Amazon” program resulted in prices for some products increasing when Amazon programmed its pricing algorithm to match the prices that certain external retailers offer to online consumers.

As a result, when prices increased, some sellers experienced a marked decline in the sales and resulting profits from products enrolled in the program. Faced with price increases, online customers sometimes opted to buy Amazon’s own branded products — particularly its private label products. This resulted in Amazon maximizing its own profits regardless of whether consumers paid a higher price for sales of products enrolled in the “Sold by Amazon” program or settled for buying the same or similar product offered through Amazon.

Yes, I read that. It doesn't address my point, which is that merchants would just leave the program if they faced declining sales.

Are you asking if a seller would leave, or could leave? To the former the answer is obviously “yes they would”, to the latter idk.

But regardless, even if they are allowed to leave the program, that doesn’t make it any less illegal.

Where would they go? Amazon is where the customers are. This is the problem.

They’d go back to selling outside of this program. They don’t need to leave Amazon to do that.

Amazon is actually fixing the price that you are selling on Amazon matched to what other sites are selling. There's no point in leaving if you're selling your widget on other platforms for the same price.

The article says so.