You are comparing the first mass-market release of Chrome extensions with the first alpha release of Jetpack, which isn't fair. A better comparison would be the first dev channel release of Chrome with support for extensions. That was May 14 2009:
... which is slightly before the first Jetpack alpha which was May 20 2009.
In any case, the issues that both extension systems try to address have been well-known for some time. It isn't surprising that multiple people came up with similar ideas on how to fix them.
ninja edit: Got timelines mixed up first time 'round
Interesting catch. Reminds me of Mercurial and Git births (in that they both were independently created around the same time to solve the same problem).
http://news.cnet.com/chrome-extensions-draw-near-but-advance...
... which is slightly before the first Jetpack alpha which was May 20 2009.
In any case, the issues that both extension systems try to address have been well-known for some time. It isn't surprising that multiple people came up with similar ideas on how to fix them.
ninja edit: Got timelines mixed up first time 'round