I’m not Catholic, either. I don’t have to be Catholic to see, that these are two entirely different situations.
The man who was excommunicated by the Mormons, had done nothing more than discuss with his family the fact, that he had decided to leave the church because of what he had learned about it. He was a perfectly ordinary private citizen, who has been attacked in a bizarre move that makes Mormons look even weirder than they all ready are.
The nuns were openly following a religious cultist. There had been a period of years where there was every attempt made to get these women to stop espousing this “Army of Mary” group, whilst all the while claiming to represent the Catholic Church. They were living before the people around them as Catholics, while denying Catholic teaching & defying the bishop. People in the community were being scandalized & their eternal souls ( as well as those of the sisters) were put at risk.
If anything, its the
opposite of the Mormon thread. The Mormon man wanted to quietly leave them, & they turned it into a sideshow. The so-called
Army of Mary were causing a public scandal over a period of years, & the bishop finally had to step in, to prevent any further attempts at spreading this heretical teaching. (Their leader
thinks she was the Blessed Mother in:shrug: a former life, for goodness’ sake!)
The case with the nuns is, if anything, more like the case when the Free Methodist Church in Canada excommunicated Benny Hinn, because of his public shenanigans. It was necessary in order to make clear that he is NOT approved by the FMC, since he is out there in front of the world making himslef ridiculous.
Acting to prevent continued public scandal is one thing.
Creating a public scandal by excommunicating someone who has all ready resigned is quite another. That’s what the Mormons are doing.
It’s not just comparing apples & oranges. It’s more like

comparing banana cupcakes and secondhand cars.