They want to say that non-whites are naturally dumber than whites. There will be varying degrees of subtlety but that's what the dog whistlers are on about.
Here's another way to say it: when you import the third world, you become the third world.
Oh. Racism.
I'm so relieved I left social media. Sadly, via democracy, even if you leave social media you are still impacted by those who use it and believe what they read.
You can take a look at the charts in the linked article and see that the scores for Asians are consistently higher than the scores for whites which are consistently higher than the scores for blacks and Hispanics. This was true in 2012 and it's true today.
If these numbers are at all meaningful for determining how much a student has learned as a result of going through schooling, then thye show that white people are consistently better than blacks and Hispanics at school and are consistently worse than Asians.