I can make a video of the world's smartest goldfish - when I say left, he turns left. When I say right, he turns right. When I say jump up and get some air, he does it. Wanna buy my intelligent goldfish?
OK, well actually I recorded my goldfish for several hours, kept repeating commands which he paid no attention to, then edited the tape down to just the occasions where his random motion happened to agree with my commands - the times he happened to go left when I said left. Maybe, I'm even sincere and honest, I believe that the goldfish is understanding me because I want so badly for it to be true. Still wanna buy my intelligent goldfish?
That's all the videos were - clips from much longer videos where Terri appears to be interacting, responding, out of hours of tape of her doing what PVS people always do, what the brainstem does when there is no brain to make meaningful motions - it just moves things around, blinks automatically, twitches the face into random expressions. When the random expression, when the random eye movement happens to match something the family is saying or doing, it goes into the video, when it doesn't, it's cut out - several hours of video went into that short clip they created. How much her family believed because they so much wanted to believe she was responding is impossible to know - but from the scans from that time and from the autopsy, they know she was blind, and her brain was gone, so she wasn't responding.
The real doctors who examined her, the independent doctors who reported to the judge - they saw her doing these things whether anyone was in the room or not, and no response to any commands, any voices, whether her mother or a random stranger. Terri wasn't really any kind of borderline case - the doctors knew for years before Michael accepted it that she wasn't ever coming back.