Both of what you're saying is "true".
There is a G13 Labs seed company who puts out seeds with G13 in the name
playing off the legendary hype of the story of how G13 came to be.
Seems the government story isn't that far off, but probably still a tall tale.
http://www.seedsman.com/en/origins-of-g13/
The government has definitely and may still have labs that they use for growing.
I remember seeing some marijuana documentary of a man who still (as of 2008
i think) was getting metal tins full of prerolled joints sent to him from the government
for his severe conditions.
Just curious but what were you told were the flowering times on your SIS? It looks pretty
sativa heavy, and hopefully some kill.