First and foremost, there are about 71 million millennials in the United States. CreditCards.com surveyed 1,000 people. It's unclear how many of those thousand people were millennials -- but whatever the case, it's obviously a fractional representation of the overall population. It's like finding a hair in your ice cream sundae, and assuming every ice cream sundae ever must contain hair now (probably because of millennials).
Secondly, the data loosely classifies restaurants, and includes fast-casual, fast food, and take-out options.
And, being the youngest generation on record here, millennials are naturally going to be the generational segment making the less overall money. This is paramount. Not only would this factor into being less able to drop tips in general, but more importantly, it also ensures that the kind of restaurants millennials frequent, would probably be ones that you don't normally tip at. Sure, when I get a slice of pizza, I'll commonly toss a buck or two in the tip jar if I have it. But more often than not, I don't.