I'm an orthopedic radiologist. A full tear is very difficult to miss and swelling/inflammation has nothing to do with it. But unfortunately there is no perfection in anything, let alone medicine. If an ACL is getting missed, it's probably in some rural town with a general radiologist, but that's not happening with D1 athletes/facilities. The practice comes in looking at partial tears which the decision for surgery comes down to the orthopedist physical exam.
It's the pain and swelling that make it hard for the surgeon to make the physical diagnosis. But that doc is right that pain can limit how the athlete is positioned in the scanner and lead to them moving which hurts the images.
The vast majority of times we get the patient already knowing the ACL is torn and it's just for confirmation (so the surgeon can charge for the knife) and to look for associated injuries (menisci, LCL, MCL) that can complicate surgery and recovery.
I've met Andrews a couple times. Nice guy. He also did my Tommy John.