The Zagg is tempered glass. The one in the video is silicone/gel.
Regardless, Samsung's statement that you should only use Samsung accessories is incredibly stupid, as it ignores the fact that someone could simply put the case/protector shown in the video on someone's S10 and the job would be done. It's still a blatant flaw with the ultrasonic sensor (apparently).
They mention they did the fingerprint registration after the gel application. I want to know what would happen if they took a proper fingerprint scan before it. Cleaned the glass, apply the gel and then try unlocking. I bet you it wouldn't work.
Regardless there's plenty variables that need to be considered. I don't think applying the gel screen on a pre-registered fingerprint will work (especially if the glass is cleaned and no remnant trace of a fingerprint). However, if the glass was not cleaned and there was a trace of a fingerprint on the scanner, I can see it being a possibility as the gel will somehow preserve it and maybe fool the fingerprint scanner.
Overall, I think it didn't take a proper scan of the fingerprint in the first place resulting in this happening. The gel probably obscured the scanner from taking a proper image of the fingerprint.