For instance:
https://www.brookings.edu/research/a-progress-report-on-the-...
"The 2012 Brown Center Report predicted, based on empirical analysis of the effects of state standards, that the CCSS will have little to no impact on student achievement. Supporters of the Common Core argue that strong, effective implementation of the standards will sweep away such skepticism by producing lasting, significant gains in student learning. So far, at least—and it is admittedly the early innings of a long ballgame—there are no signs of such an impressive accomplishment."
That's the context of your cherry picked sentence, that even a foundation expecting no benefits found them, and is left with trying to downplay those findings.
And that's to be expected, a long tail of continued improvements as the students get older and the benefits of increased numeracy in higher math pay off, and as we get better at communicating to K-5 teachers (who, frankly probably didn't have the best math background either) start grasping the meta point of what's being taught.
> States that more aggressively implemented the CCSS registered larger gains from 2009-2013. That’s an optimistic finding for CCSS.
Even they find that clear data that states implementing CCSS have better outcomes, only a few years after implementation, that correlates with how aggressively CCSS was rolled out in the state.
I provided this source because they're ideologically opposed to CCSS, but even they admit that there's a positive effect early in the roll out. The rest is them bending over backwards trying to explain why their own data doesn't actually matter. In short I cited them because it's one of the best cases for the antithesis of my argument and it still proves what I'm saying.
That's the idea, but there's no evidence of actual improvement in, say, student math skills.