They're very strong supporters of him, by the way.
Here's the problem: Sanders is a social democrat, not a Democratic Socialist. That's a big difference.
From part of their website:
http://www.dsausa.org/toward_freedom
Socialists therefore argue that private corporate property is not only wrong, but also nonsensical. Wealth is a social creation and should be controlled by society as a whole.
Literally all of socialism depends on the democratization and non-private ownership of capital. I mean, I am all for Sanders at this point. I'm all for him embracing the term socialism (though it took me a while to come around on that). But can we not pretend that socialism and capitalism are in any way compatible?
Also, Democratic Socialism does utilize a huge amount of Marxist theory. Perhaps you meant to differentiate between Trotskyist socialism and DS?
Now, if you want to say that
Sanders' brand of Democratic Socialism is all of the above, go for it. But this is just straight up false.