The original comment quoted above said “back-up school rather than a first choice school.” – first choice school is elsewhere, making WUSTL a back-up to that first choice. Considering the low rate of kids who applied ED compared to peer colleges and high rate of RD admits choosing to attend elsewhere compared to peer colleges, I think this is a fair assessment.
Of course this does not mean that WUSTL is a backup = safety. WUSTL is extremely selective, with a low admit rate. It would obviously be a poor choice for nearly anyone’s safety. Being more selective than the vast majority of colleges, it would likely get the majority of cross admits for the vast majority of colleges. However, for assessing whether a college is a backup for students who apply, you need to consider the other colleges that the student who applies to WUSTL also tends to apply to and/or tends to be interested in, not just a random list of all colleges in the United States.