Probably a combo of the pictures, the original post (with the comments of the OP being surprised) and other coins of this series in the AU 55 grade. Based on the image, I wouldn't be surprised by any of the AU grades (maybe outside of AU 58). Do these eBay examples at AU 55 look significantly different? https://www.ebay.com/itm/1837-N-13-...615119?hash=item3ab64d304f:g:FywAAOSwT5tWQ5~C https://www.ebay.com/itm/1838-Coron...329399?hash=item262dcf7af7:g:OQ0AAOSwIHpd6r~0
They don’t look much better with respect to luster but the reverse of the coin in question looks completely flat. I don’t collect early copper so I don’t know how the different TPGs treat wear & luster for AU coins. Based on the guesses, they must be pretty lenient.
I'm not big on early copper either, but I do know it can be quite different from other coins. Earlier this year I won an auction for a half cent in a PCGS MS 64 holder with CAC. In hand the coin was also fairly flat. It was a true UNC, but the luster was nothing like I'd learn to expect on silver coins or more modern copper. I ended up selling that coin at a show for a slight loss and have been more careful with early copper since.