hadn't heard of this project before, but their github does include instructions on how to claim it even in a network-less dedicated docker container if you're truly paranoid.
https://github.com/fluencelabs/dev-rewards/blob/main/MANUAL_...
https://github.com/fluencelabs/dev-rewards/blob/main/MANUAL_...
If they genuinely wanted proof of account ownership there are other ways to do it. OAuth, for example.
> I wasn’t able to confirm that this is what actually happens
So it's a baseless accusation, you can see exactly what happens.