I disagree with this assessment of silicon based life. While it is true that Earth's crust is mostly silicon, it's also at a temperature that would make metabolic processes using silicon very difficult until you get down to the mantle.

It could be that silicon based life is in fact constrained by temperature and can only arise in planets where the mean temperature is in the thousands. This could in fact mean that we do have a parallel silicon based biosphere underneath our feet. There is obviously no evidence for this whatsoever but then again, there is no reason to assume that the occasional leakage in the form of volcanic eruptions would leave any trace that we could use to deduce that these samples were once living entities.