At the time I had assumed that Automatic Duplexer was the stat you were looking for. Now I am not so sure. It could be referring to the normal duplexer along the printer paper path and not the ADF. Im not sure why the duplexer would have a different paper weight limitation than the rest of the trays but perhaps that is how the parts are designed.
I wouldnt normally think the ADF would have paper weight restrictions because all it is doing is scanning the paper. As long as it can bend around the rollers in the ADF then it should work. Paper weight is much more important when printing because the paper needs to be charged for laser printing and it may be too close for inkjet printing and gunk up the print heads.
Might be best to chat with HP for confirmation on this one. Id hate for you to invest in such a nice printer and not have it perform a critical feature like the one you are describing.