Over the years I have always used Dijon mustard when making salad dressings, etc.
I always assumed the fine grind of the mustard seeds would be the thing that helped with emulsion.
The other day I was out of Dijon and instead used an old school whole grain mustard (Maille Old Style) when making a salad dressing.
Weirdly, it worked as an even more efficient emulsifier than the usual finely ground Dijon.
Seems to indicate that it isn’t the fine grind of the mustard, but some other factor which is aiding in emulsion of fats.