The very first phrase of the article talks about open source investigative techniques, not about open source software, both are very different things.

'Open Source' in this context is a little different from the software, although I think they are also working on building open source tools to help investigate.

My understanding is that 'Open source research' has similar principles of being accessible and verifiable by anyone, as opposed to being a closed process based on trust.

Pulled this from an article: https://www.bellingcat.com/resources/2021/11/09/first-steps-to-getting-started-in-open-source-research/

Open source research has “come of age”, according to a recent article published by The Economist. What was once the niche realm of a relatively small number of individuals with free time and obsessive internet habits is now informing research and journalism in a wide range of fields and institutions. It’s hard to imagine a better time to roll up your sleeves and set off on the path of the open source researcher.

The promise of open source research is that anyone — not just journalists or researchers at select institutions — can contribute to investigations that uncover wrongdoing and hold perpetrators of crimes and atrocities to account.

The organization is pretty interesting as well. Pulling from Wikipedia because it sums it up nicely. I personally find it impressive how much they've accomplished: https://en.wikipedia.org/wiki/Bellingcat

Bellingcat began as an investigation into the use of weapons in the Syrian civil war. Its reports on the Russo-Ukrainian War (including the downing of Malaysia Airlines Flight 17), the El Junquito raid, the Yemeni Civil War, the Skripal poisoning, and the killing of civilians by the Cameroon Armed Forces have attracted international attention