And realistically, since deepfake detection will inevitably be more expensive than captchas or antivirus scanning, this will be adopted by human-in-the-loop organizations for critical processes where threat scoring or moderation is already being applied.
That said - Reality Defender, please train your system on diverse human data sets, do not release models where ethnicity or gender (including gender identity) are nontrivially correlated with deepfake score, and have processes in place from day 1 to allow users to report suspected patterns of bias. The kafkaesque "prove you're not a bot" scenario envisioned by the parent poster is one thing for holistic human-in-the-loop verification processes, and another thing if it suppresses minority voices and minority access to government services.
It's like ADA Compliance lawsuits. I can't prove the AccessaBe or other "ADA Compliance" web tooling are generating these lawsuits, but their company would not exist without them. Why wouldn't they want more lawsuits?
So back to Reality Defender, why shouldn't we doubt Reality Defenders positive or negative results? There would need to be a period of verification and testing that "proves" within a reasonable margin that it works.