This view is based on some doing some basic research on both Sander's and Warren's websites. They both have proposed systemic changes that will tackle economic inequality. Progressive taxation, a wealth tax, a higher minimum wage, etc. It's mostly about reversing the past few decades that reduced taxes on the wealthy and allowing wages to stagnate. I don't see anything radical there. Anything too radical or against the constitution would be challenged in the courts which are currently conservative. So I think all this worry about Sanders and Warren is not based on anything they've put forward. It's more of a knee-jerk reaction to change. Change that a lot of people in the working class would like to see and benefit the most from.
I would suggest that when discussing one country's politics, using another country's standards can be a little misleading. Another country being much more to the left of the United States doesn't change the fact that policies like Sanders' would represent a major shift to the left *for the United States*.
For perspective, think about doing the opposite. Let's say Norway had a conservative Christian presidential candidate who wanted to slash social programs and taxes. He might be labelled a reactionary, but we could in theory say 'well actually he's not all that right wing, he's actually only a little bit to the right by American standards.' The fact that he might even struggle to fit in with Republicans probably won't matter to Norwegians - electing him would represent a huge shift to the right *for Norway*.
All that said, right wing media does have an explicit political strategy of labeling anything to their left socialist or radical, so I don't think you're off base in general, but I do think it's appropriate to say that the country's political climate is more relevant than the globe's political climate when it comes to how much of a shift one candidate might represent; the United States could in theory be like Norway and Norway could in theory be like the United States but that kind of change would involve a massive political and social undertaking by either country, which would be relevant in a political discussion and is the exact kind of point that's getting dismissed when one says "Candidate X isn't really all that radical; he'd be a moderate in Country Y."
No counter-arguments have been posted yet.