I'm profoundly deaf, and my writing is fine... As in both GCSE and A-level equivalent English. However, I learnt to speak before I lost my hearing, so this gives me a big advantage in speaking and writing compared to a lot of deaf children. I don't use sign language, I just lip read.
I think the issue here is that this person has sign language as a first language. I have a friend whose mother is deaf, and she has been communicating with her in sign language all her life. She didn't learn English until she went to school because all she knew was sign language and right now her English is fine, but not perfect.
Some other posters have also mentioned that sign language uses a grammatical shorthand "My name Tom" "I live London" etc and for some deaf people this may transfer into writing.
HOWEVER, there is no reason why a deaf person can't have perfect writing. I think this guy in question might have a few other problems - dyslexia or mental problems maybe? I'm not a doctor though.