I think you are right.  It's a legacy of learning to type at a certain time and place.  I distinctly remember my typing teacher (teaching us to use an IBM Selectric typewriter) telling us to put two spaces between sentences.  This probably dates Satoshi (and the rest of us) as having learned to type with an older typewriter standard, OR from someone who taught an older typewriter standard.   More conclusively though it places Satoshi as having learned in an English speaking country.  Apparently no non-English speaking countries do this double space thing, according to: http://www.ditchwalk.com/2011/01/19/two-spaces-after-a-period/ .  But no one suspected Satoshi as actually being Japanese anyway.