According to the linked article, Their whole purpose was/is to “think 10,000 years in the future instead of quarterly”, so they only need 5 digits for that goal.
This was the first I'd heard of Long Now, but upon reading the article, I had assumed the leading 0 was representative of the infinite continuum of 0's in-front. But all the repetitive 0's had been slimmed down to one, like :: in IPv6