Bad numbers, bad math
The arithmetic in this article is terrible. First, it starts by saying there are 10m bacteria per cm2 (155,000 per in2). Note the ratio, 10m in a small area vs 155,000 in a larger area.
The rest of the article appears to use a ratio of 2.54 cm2 per in2. This ratio is correct for converting between cm and in, but not the squares - for that, you must square the conversion number, reaching something around 6.45.
So if there are 10m bacteria per cm2, then there are 64.5m bacteria per in2. However, this is inconsistent with the rest of the article, wherein the number 203,000 is given as higher than average, the concentration in an average armpit. (203,000 per cm2 is not 516,000 per in2, as stated, it's around 1,310,000 per in2.)