An IQ of 0 corresponds to 6.66 standard deviations below the mean. It's functionally unmeasurable (when a person is too stupid to take even the tests specially calibrated for people with exceptionally low intelligence, their IQ is too low to quantify,) but an IQ of 0 does not correspond to "zero intellect." "Profound mental retardation" has no defined lower limit, only an upper one. You can also check the link you provided yourself, and you will find that it does not actually make any mention of a lower limit of zero, it defines profound mental retardation as being "<= 20-25"
An IQ of 100 does not correspond to 100 Intelligence Units, where an entity with zero Intelligence Units has no intelligence, it is simply the defined average of the population, and IQ tests are re-normed to have an average of 100 when the average intelligence changes. IQ points are meant to define where a person falls on the normal curve of human intelligence, not quantify intelligence on an absolute scale.
Recently I stumbled upon Richard Carrier's essay "Are We Doomed" (June 5, 2009), when asked to comment about the Singularity, said the following:
What do you think?