How does using a pen and paper encourage proper grammar and spelling? It doesn't, you do that. When taking notes, you abbreviate, when writing formal communication, you use proper grammar.
Keyboards don't change this.
The issue is that touch screen discourage proper typing. Lets say someone gives you ten pages of handwritten text and asks you to email a soft copy of the text to them.
You have the following in front of you, a desktop with a full keyboard and monitor, a laptop, an iPad with a bluetooth keyboard, an iPad without one, an iPhone.
Which would you or anyone else choose for this task and in what order of preference? Why?
Now lets say you have to write an essay on a topic in 10 minutes and the limit is 3000 words. How better would the essays come out on each device?
Now apply the same logic to writing emails or even online comments and the GP's statement is very true.
The choice of which keyboard is available definitely influences writing and even content.
Assume that suddenly you were unable to talk for a week and had to communicate through a device like Stephen Hawking's. Would your "talking" be the same as now? Why?