The Bernie Sanders being a Communist accusation is actually a very good comparison.
People are pretty much saying. Bannon calls himself alt-right. White nationalists call themselves alt right. Therefore Bannon is a White nationalist.
It's just like saying, Bernie is a socialist. Stalin called himself a socialist. Therefore, Bernie is a Stalinist.
Clearly, Bernie and Stalin have different definitions of socialism and Bannon and the White nationalists have different definitions of the alt right.
The fact that you are having to defend him by saying not all socialists are communists though on its face is a reasonable argument, has really moved the goal posts when it was that anyone who was a socialist was unacceptable in the last cycle.