The advice in the article is too broad because there are times when the comparison is not meant in the truthy/falsy sense. For example, strcmp and friends return 0 if the strings are equal, less than 0 if str1 is lexically less than str2, and more than 0 if str1 is lexically greater than str2.

So if(!strcmp(str1,str2)) is a valid way to check for equality, but I think the negation makes it confusing; it is tempting to intuitively read it as checking if the strings are not equal. if(strcmp(str1,str2) == 0) is the better choice.