Above is an illustration of the “No True Scotsman” fallacy from Ali Almossawi’s An Illustrated Book of Bad Arguments. How the fallacy works:
A general claim may sometimes be made about a category of things. When faced with evidence challenging that claim, rather than accepting or rejecting the evidence, such an argument counters the challenge by arbitrarily redefining the criteria for membership into that category.
For example, one may posit that programmers are creatures with no social skills. If someone comes along and repudiates that claim by saying, “But John is a programmer, and he is not socially awkward at all”, it may provoke the response, “Yes, but John isn’t a true programmer.” Here, it is not clear what the attributes of a programmer are, nor is the category of programmers as clearly defined as the category of, say, people with blue eyes. The ambiguity allows the stubborn mind to redefine things at will.