This sounds like a plausible motive. There's a handful of huge games using Unity that together generate upwards of 20bln annually, such as Pokemon GO, Honor of Kings, and Genshin Impact. I'm guessing their soundbite of "developers being excited" over the change is with those behemoths in mind, and not the other games that are barely making ends meet.
These particular successful games are still going to save millions compared to Unreal, while the change means that everyone else would now be more profitable with Unreal. Pokemon GO alone would have made Unreal $100mln+ a year with the 5% royalty, while paying considerably less with Unity's new scheme. Even at a billion downloads it's only $10mln in comparison.
It sounds like Unity is shifting their focus from the long tail where they were successful charging annual subscription fees (and ads), to the head that's generating orders of magnitude more revenue.