It is my opinion that Silversea's dress code is unclear on purpose, for reasons stated in cruiseej's excellent summary above, in which the last paragraph is right on the money. They could publish a concise, clear dress code very easily if they wanted to, but they do not.
They want to market to all potential cruisers. and it is only a small percentage of guests onboard (as opposed to the percentage on Cruise Critic) who really care what anyone else is wearing.