When Postman refers to written word he refers mainly to books. The article mentions the increase of hours spent reading text as a result of texting, social media etc. This form of written word is actually close to Postman's concerns.
It's written word, but its metaphor is the same as the metaphor of mediums like TV.
I agree in part. The texting, social media, etc. has the same disjointed tone as the "now, this" attitude of television that Postman wrote about. However, he had a specific concern with images versus text (whatever kind of text) -- namely, that they pass too quickly for people to dwell upon and can never advance an argument in the way that writing can.
To be honest, I think this is a general weakness in Postman's work. He talks a bit about the difference between pre and post-telegram writing, but he lionizes all 18th century writing despite there being tons of different forms (pamphlets, books, novels) that don't have the same intellectual value imo.
It's written word, but its metaphor is the same as the metaphor of mediums like TV.