Kwanzaa is a joke and I know NOBODY that takes it seriously. In fact, I can even disrespect it in my otherwise PC workplace. The reason is simple: As you pointed out, it was created by a man in 1966, Dr Maulana Karenga.
What that means is that when I disrespect Kwanzaa, I am not disrespecting a people or a religion. I am disrespecting the equivalent of the Rush Limbaugh Bake Sale. It is not a politically or religiously protected thing. Sure, there are people that would be offended if I disrespected either event, but neither event (or belief) crosses the line into the politically or religiously protected.
And Kwanzaa IS pretty funny, actually.
Kwanzaa is a fake holiday.