I would be interested to know the author's definition of grammar. He seems to be using the common yet vague definition of grammar, something along the lines of "the proper arrangement of words in a phrase or sentence." However, from a critical/academic standpoint I always thought grammar was more general. Wikipedia, m-w, and answers.com all allow for "grammar" encompassing essentially all structural rules of a language, including semantics, syntax, morphology, etc.
Also, I specifically disagree with his number 3. I have never heard or seen "I can do that easy" in formal speech or prose. He seems to be arguing against grammar Nazism, which I can agree with, but that doesn't mean that a perfectly acceptable and understandable sentence may still be ungrammatical. "I can do that easy" is ungrammatical.