> ?!? Why bother defining a framework for measurement if you're just going to substitute your own judgment? Perhaps the framework didn't capture something important -- like the absolute number of people looking at any given post, concurrent updates, etc.

Just guessing: the benchmark tells you the time complexity. It doesn't tell you the space complexity. The author is optimizing between the time- and space-complexity of the solutions, with the time-complexity benchmarks as an input. (Also, at a scale the benchmark doesn't reach, space-complexity starts to affect time-complexity, as large datasets become less able to be hot in disk cache.)