I saw this post and was searching for it, couldn't find it. Convinced myself that it was retracted.
We had similar. They see the book lists or something else with a grade level and they comment. I think my kids just want to know that they are indeed in the right place. When dd gets frustrated, she says, "why did they even make this for first graders". Doesn't matter what level it's at.
When we are picking out books, I say, "you enjoyed x book. I looked it up and they say that's a 3rd grade level. So, you can try books from this list". If pushed, I blame it on the list makers. I say they want to be careful not to put books that are too hard on a list for someone who is just learning to read at K or 1, but since dc already read, it's OK to read from that list. I don't say those books are unchallenging or not interesting, or make any judgement. My dd who skipped K and is going into 2nd, still likes to read Henry and Mudge and Mr Putter and Tabby, and even baby board books in addition to her 4th grade level books. She finds them interesting. So, I guess I go more along the lines of "this is how they do it, and this is what I think would be good, but I could be wrong, let's try it". Rather than, "you are past that". I want my kids to feel free to follow their interests and not be bound by a list someone else created. I rightly or wrongly believe that it will help them become more confident and accepting of who they are. But, then I have a child who pushes herself and doesn't need me to convince her to try challenging things. Back when she had a confidence crisis and was afraid to try, I did force her to read things at her level to get her out of a depression, but I didn't stop her from reading "easier" books either.