I am a retired teacher of English in secondary school and I have never heard this assertion before. "She reads us a book." is perfectly good and standard English across Canada and I expect all of North America. So is "She reads a book to us." But one would not say "She reads to us a book." That sounds stilted and awkward.
I think it is a bit of a bug too because he/she is not the error in your translation. Both are correct. However your verb is incorrect. It should be "He (or she) reads us a book." "He read us a book," is past tense and the verb in this sentence is present tense, so it requires "reads" rather than "read" when translated into English.
Usually if I write "he" when the program uses "she" it doesn't tell me I am wrong. It accepts the answer and offers the alternate as "another correct answer" Maybe you got this weird response because of the actual error in the verb tense, but then the program made the wrong correction.