Since before it was the CIA. Open Source in this context isn't about software licensing, it's about gleaning everything you can about an adversary from publicly available information without infiltrating anything.
A non-intelligence example of this would speculating on what products a company is working on based on their job postings.
To bring it full circle, one can glean from this posting that the CIA is looking to hire someone to troll the internet for folks to target for further intelligence gathering. Business as usual.