Ymr – a better metric would be to look at the classwork assignments,and not just the 2 cherry picked papers they found which fit their narrative. I taught at a satmar-type school, and i had kids writing cute poems in, i would say, 4th grade level english, when they were in 6th. That’s pretty good for a 2nd language. Their math skills were very sharp; they did mental math faster than i did.
And that’s just English. Their first language, which they are all literate and fluent in, is yiddish.
My kids wrote entire grammen in yiddish, short stories…their kodesh homework included writing assignments…the times omits the language arts that is definitely gained in yiddish.