which suggests that you believed that "ten times as smart" must map to "ten times the IQ score."
To the extent that IQ is the only quantified form of intelligence yet known, yes, that's absolutely true.
, but ordinary thermometer readings aren't in Kelvin,
Celsius is Kelvin + a number. Fahrenheit is Kelvin + a number + ratio conversion. This is a total non-starter for your position.
and neither do IQ tests measure from zero intelligence at zero IQ.
0 IQ however does have an intelligible meaning. This again is a total non-starter. Nothing in the observable universe exists at 0K. We "measure" above 0K. No intelligent being actually has 0 IQ. We "measure" above 0 IQ.
Kelvin is quantified temperature relative to absolute zero. IQ is quantified intelligence relative to zero where the units are adjusted to fit the current history of measurements.
mapping "ten times as smart as IQ 25" to "IQ 250" would be rather like mapping "ten times as hot as the reading of 12 degrees C on this thermometer" to "120 degrees C."
No, it would be exactly like asserting that 120K is 10x the temperature of 12K.
If you'd read the link that I originally gave, you'll note that the "profoundly mentally retarded" goes from zero to twenty-five.
Zero, in this case, means without intellect at all.
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 profoun...
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?