It's a response to someone saying "you can't draw any conclusions of IQ significantly before 1950 from how the line behaves after 1950", and it says "And that’s because IQ is a statistical distribution, not an absolute measurement of intelligence."
This seems like a non sequitur to me. Am I missing something? (Bear in mind that the 'line' under discussion is an increase in unstandardised scores.)
Extrapolation is the most questionable statistical tool, and while extrapolation ad absurdum is a way to show a formal predicate logic argument to be incorrect or underspecified, it is an almost fully general attack against real datasets, which basically always have some trend line that ultimately passes sensible thresholds like zero bounds. Showing this, however you form the trend line, is not saying a whole lot.
Extrapolation prior to 1950 is not a very useful tool to evaluate intelligence trends, and this is entirely separate from the periodic recalibration of IQ tests to keep the average at 100 (however many correct answers out of 1000 this corresponds to).