Actually, data indicates that in a gathering of 10,000 people in Orange County, you have a 99% chance of being exposed:
https://covid19risk.biosci.gatech.edu/
Also, as to whether this person pulling down a mask is infecting others, you assume that everyone in that crowd has an equal chance of carrying the virus. But, some have a higher chance than others, depending on their previous behaviors. Pulling down a mask, in an indoor space, at an amusement park, with people close by, indicates a risky behavior. If they are engaging in a risky behavior here, chances are they have engaged in a risky behavior previously that could have put them in contact with the virus (past behavior is the best predictor of future behavior). So, there will be people with COVID at Disney and those that engaged in risky behavior are more likely to have been exposed. We should not minimize that what they are doing is unsafe. I do see the point that shaming in an online forum is rather pointless, but there needs to be a way to communicate to Disney that unsafe behaviors are taking place.
All of this is a moot point though. Proper face covering is required at all times except for swimming and when sitting down to dine. This person does not have an appropriate face covering at that moment and is violating the rules they agreed to when they set foot in Epcot.