How bad are these scores? Very bad. If you know half the letters in the alphabet, we don’t call you “half literate.” We correctly call you illiterate.
Seems to be a lack of civility here and/or an appeal to emotion. In this section the author targets the audience and tries to make his point by invalidating intellectual ability.
The general pattern is that grown-ups have shockingly little academic knowledge. College graduates know about what you’d expect high school graduates to know; high school graduates know about what you’d expect dropouts to know; dropouts know next to nothing. This doesn’t mean that these students never knew more; it just means that only a tiny fraction of what they learn durably stays in their heads.
There seems to be a lack of civility throughout the argument, using many negatively charged words to imply that the public retains no knowledge.