But they can. They do when the tertiary is emphasized over the auxiliary. "ISFP" doesn't necessitate that he uses his S function more than his N function; it just indicates an Fi+Se+Ni+Te functional grouping. The order of priority is up to the individual, though--some people have learned to develop the tertiary above the auxiliary.
INTJ makes more sense than INFP.
INFP doesn't work because he doesn't use Ne or Si. His posts are ridiculously Ni.