No, they don't believe that the center is overall best because they don't care about anybody else.

Progress means you aren't there yet, you always have further to go. "Progressive" isn't just a label of what their views are, but a value judgement that their views are best. This is what they call themselves.

So the best liberal is the most extreme liberal; "far left" for a liberal is the center and the farther left you are the closer to "best liberal" you are. It's a total inversion of normal reality just like their other views.