If I engineered the tech I would be much more fearful of the possibility of malice in the future leadership of the organization I'm under if they continue to keep it closed, than I would be fearful of the whole world getting the capability if they decide to open source.
I feel that, like with Yellow Journalism of the 1920s, much of the misinformation problem with generative AI will only be mitigated during widespread proliferation, wherein people become immune to new tactics and gain a new skepticism of the media. I've always thought it strange when news outlets discuss new deepfakes but refuse to show it, even with a watermark indicating it is fake. Misinformation research shows that people become more skeptical once they learn about the technological measures (e.g. buying karma-farmed Reddit accounts, or in the 1920s, taking advantage of dramatically lower newspaper printing costs to print sensationalism) through which misinformation is manufactured.