The stat is skewed wildly by immigration. The literacy level of native born Americans is higher. The population of foreign born adults is nearly 20% of the total adult population, and as you can imagine many are actively learning English.
See: https://www.migrationpolicy.org/sites/default/files/publicat...
51% of native-born adults scored at Level 3 or higher. This is considered the benchmark for being able to manage complex tasks and fully participate in a knowledge-based society. Only 28% of immigrant adults achieved this level. So yes immigrants are in trouble, but it’s still a huge problem with 49% native-born below Level 3.
Seems like the standards have changed over time?
It's fine if we want to change it to "sufficiently master language to do a white collar job", but if the standard changes we shouldn't be surprised fewer people meet it.