My answer would be that 'sit' is just another way to say 'be' (on or next to something). I wouldn't say it's really anthropomorphic at all -- the urn was not being personified, it was just there. It certainly wasn't lying, because it's not a flat shape that can spread out on a surface without rolling or moving (a garment, book, newspaper, etc.), and it certainly wasn't standing, because it's not a tall shape that can stay upright without tipping or falling (a tall shelf, rack, sign, etc.). It's just a neutral shape, and it was somewhat upright (it has a top and a bottom), but not tall. Makes sense to me. (-;
My impression is that with things, not people, German actually uses specific verbs ('liegen/legen,' 'stehen/stellen,' 'sitzen/setzen,' and 'stecken') considerably more often than English, which often just uses one general verb for everything ('be/put').
So if English does occasionally use a more specific verb, I would have thought that German speakers would be the first ones to understand that kind of usage, even if the specific case is slightly different.
But sometimes it's just where each language draws the boundary between the different shapes that's confusing. For example, this plate (or bowl?) of dirk's. I think that plate would actually be sitting on the table in English, because it doesn't have a flat surface against the table, and it's not tall. So I might as well ask, How the heck can the the thing be standing up in German, did it suddenly grow a pedestal and little feet? But the lamp, now: Obviously it should be standing on the table, because it's tall and does have a pedestal. Sitting? Hmmpf. Totally unfair. (-;