I also thought the S stands for Switch, honestly. I think both of you have a point.
I'm not sure about that point though.
I told him this earlier, but he either didn't see my comment or ignored it.
Dragon Quest XI S received a subtitle to highlight that it's an enhanced version with a lot of additional content that separates it from the original 2017 game in a substantial way.
But if Atlus were to announce a P5 port to Switch around this time, it would naturally be the same game as Royal. Which is why people didn't think they'd give the same game two different titles. A couple of potential Nintendo DLC costumes wouldn't warrant a name change imo.