Huh? If a deaf person needs to look at a screen to see the signs then they could just as easily look at a screen to see the live speech to text of what's being said

Why not just do speech-to-text and display the text???

Mentioned in the article: this supports many african languages, including african sign languages. (There were plenty of apps that already did this for, say, ASL.)

Relatedly: it's possible to be deaf and illiterate. Similar to being able to speak but not read, these folks are much better served by sign language translation over text. (I can't find data on rates quickly, but see here for discussion of the general phenomena. It's harder to teach the deaf to read, so easier for them to fall between the cracks.)