The term literally is sexist because it implies it's only bad when men do it. These days it's used to describe any time a man explains anything. It's lost any meaning it may have had originally.
White knightism is sexism in its own right too, because it brings to the table the assumption that women are weaker and need protection, thus not equal to men,
I never said it doesn't happen. I said it's overblown.
Online, literally anything a man has ever said seems to be described as mansplaining now. Offline, I've barely heard anyone complain about it - only talking about car mechanics I believe. And I've got some people in my circles who are pretty vocal about gender dynamics issues.