Indeed, you're right. I thought at its most basic it is just egg whites and oil, but googling that specifically says mustard is part of the recipe.

So it's still a mystery why McDonalds doesn't just call it Mayonnaise when the ingredients are pretty much identical to every other mayonnaise out there, including Hellmann's REAL Mayonnaise.

Maybe it's one of those "It's only X if it comes from the X region" type issues. It just makes it sound suspicious for no discernable reason. Like buying a bottle of "Wet 'water' style beverage."