I think the idea is that people generally say "low fat" to mean healthy (even though that's not true). The "+ sugar" just makes it doubly more unhealthy. The drink itself is the oxymoron.
Ah, so it makes sense as long as we assume the words being used don't mean what they actually mean. Ok.
/s
It's true but probably misleading to the people who the advertisement line is directed to. Of course, that's definitely the intention. It seems odd that anyone would want to have a low-fat diet voluntarily.
Lots of diets are odd. Plus it's just a coffee beverage, not soylent; it's not supposed to be your whole diet.
Personally, I prefer the taste of low-fat milk.
I prefer skim milk in coffee because fat doesn't mix well. I also don't like sugar in my coffee, but that's an independent matter.