I've seen this cake at the local store some years back (and can still find this cake in stores now). I too noticed it was a playmobil treasure chest from the get-go.
IMO, this is not simply a case of a similar-looking chest; it has the exact same shape, the only real difference being the wood grain detailing on the planks. In my book, it would qualify as a knock off of the original mould.
However, I doubt even if made aware of it, that it would be worth it for Playmobil to expend the time, money, and energy on a lawsuit for such a trifle accessory. Now, if it were a single Klicky, or perhaps several more parts, I can see a lawsuit being worthwhile.