From the screenshots it looks like they just translate English to sign word-for-word, skipping words that are harder to translate. So it's not translating to (American) Sign Language, which has its own grammar and is not just English with function words removed.
It might be neat if they fingerspelled the missing words. IDK as to irl applicability either way tho
I found Sofie Beier's ATypi presentations to be in depth as well as humble about the lack of hard science in typography, but haven't looked too much into actual papers. https://www.youtube.com/watch?v=xAzFiyqNO4w