I think you may have misunderstood the answer to the question you cited. It says that light reflected from a silvered mirror will be mostly unpolarized. This is true whether the silver is on the front or back surface. There is a very slight polarization due to the less than ideal properties of the silver.
The front surface of a back-surface silvered mirror will reflect highly polarized light, but whatever gets past the front surface will be almost perfectly reflected by the silvered back surface.