They specified the geometric mean.
The arithmetic mean (what you're thinking of) of 1 and 100 is 50.5.
The geometric mean of 1 and 100 is 10. It gives a sense of the average magnitude.
They edited the comment, previously it did not mention geometric mean.
The geometric mean seems to be the natural mean for relative comparisons between lengths, because the mean of (Planck length, observable universe) is clearly very different from the mean of (house, observable universe).