Metal has a lot of potential issues in the microwave (electric charge buildup + arcing, and microwave reflection). There are too many variables to make general statements like "such-and-such metal is safe" or "smooth objects are safe" with confidence, hence the sweeping guaranteed-safe blanket advice to not put metal in the microwave.
The reason it didn't cause issues for you is likely the fact that it was insulated in a plastic container (thus preventing arcing to the microwave walls / magnetron) and that you also had some water in there to absorb any excess reflected energy (and maybe electrical energy through physics that I don't understand, but don't quote me on that one).
In fact a cursory search just happened to turn up this paper regarding sterilization of metal objects in a microwave, which mentions:
Arcing back to the magnetron and damage to the microwave oven are prevented by placing a radar absorbent material within the oven and with proper insulation of the item to be sterilized
And you've got both of those things: A radar absorbent material (the water) and the insulation (a perfectly sealed plastic container).
The type and shape of your metal object is probably unrelated to the success of your accidental "experiment".