What I think is strange with stuff like this that you should be able to come to that conclusion without technical knowledge. Just the fact that everyone runs AI on GPUs and NVIDIAs stock skyrocketed since the AI boom should tell you s.th..
Did OP really think his fellow humans are that moronic that they just didn't find out you can plug in together a cuple of rasperri pis?
Nobody thought an RPI cluster would ever be competitive, and Geerling never expected anybody would. But it's fun to play "what if" and then make the thing just to see how it stacks up and that's his job. Any implication or suggestion of this being a good idea is just part of the story telling.
I think environment variables are recommended to pass configuration parameters, and also secrets, in containerized applications managed by container orchestration systems.
By design, other processes cannot inspect what environment variables are running in a container.
Also, environment variables are passed to child processes because, by design, the goal is to run child processes in the same environment (i.e., same values) as the parent process, or with minor tweaks. Also, the process that spawns child processes is the one responsible for set it's environment variables, which means it already has at least read access to those secrets.
All in all I think all your concerns are based on specious reasoning, but I'll gladly discuss them in finer detail.
(inb4: container env-vars are isolated from other containers, not from processes on the host system)