I only take new-ish cars to a third party if I need something the dealership doesn't specialize in (body work, tires, etc.). I'm not that price-sensitive about service, and I like the dealership experience (nice new loaner cars, all the parts are on hand, etc.). So I'd wonder what it is that drives people to Your Mechanic, and if that affects the volume and type of problems they see. For example, maybe people are more likely to use them if they have just received a large estimate for a major problem from the dealer. Or people who are more price sensitive go there, which might skew the mix of models they see. And so on.
While this is interesting data, I have a hard time considering it representative of reliability as a whole without some additional context.