Supported typing/facilitated communication is widely regarded as a pseudoscience. Studies have shown that FC is unable to produce answers not known by the facilitator. FC proponents believe that autistic individuals have the same linguistic ability as neurotypical individuals, and difficulty speaking is merely a motor issue.
As someone with autism, I can tell you: my brain can barely keep up with conversation. It's not a motor issue. I have to actively think about appropriate word choice, how to structure sentences correctly, and neurotypicals don't. If I don't take enough time to finish the sentence in my head, the intonation is wrong, I'll skip words, put them out of order, and just generally be unintelligible.
FC, like many other 'theories' surrounding autism, are made by people who have put years into researching autism, but have never thought to ask an autistic person anything about their experience.
Source: https://en.m.wikipedia.org/wiki/Facilitated_communication