You think that Democrats have not moved in the past 10-15 years? You think that the idea that they have is merely Republican propaganda? I think you haven't been paying attention.
Or maybe you were paying too much attention. 15 years ago, AOC wasn't even old enough to vote. Were there Democrats in the House who held the same positions 15 years ago? Perhaps so. They didn't have the press attention that AOC gets, though. So they may have been there, but in the eyes of the public they weren't "the position" of the Democrats. Instead, they were fringe.
Now AOC gets as much press coverage as Nancy Pelosi (maybe more, before impeachment). To the average person, AOC and people like her now represent the Democratic Party. That's a massive shift, at least in perceptions, if not in actual position.
And I think the actual position has shifted, too. There are considerably more of the farther-left people like AOC, both in congress and running for president. The median Democratic officeholder position has shifted left. (Or so I strongly suspect. I assert it without statistical evidence.)