You read it wrong. It didn't say he couldn't read or write. It said this:
If the meeting is less than five minutes, the gist of it can be communicated through a short written note or text message, MacDonald said. But if it's longer than five minutes, he said he would need an interpreter "because it's just too complicated."
Like many deaf people, MacDonald doesn't fluently read or write English. Asked why MacDonald couldn't learn to read English better, Foster explained that a major part of learning a language is to hear it, which MacDonald can't do.
Read more at
http://www.philly.com/philly/news/20151210_Deaf_employee_sues_UPS.html#38U7J4S3mP5pcmkt.99