One possibility: unemployment is defined as someone who is not working AND actively looking. If wages at McD went up to $15 an hour, some people who were not actively looking may enter the workforce.
That is certainly true. However, if I had said 'employment is not decreasing' instead of 'unemployment is not increasing' it would still have been a valid critique without being vulnerable to that response, however.
Yep. FYI economists use the term "labor force participation rate" for this concept.