It sounded like he got no support from his family and "ran away" as soon as he was able. But, he probably could have earned a GED. But, that takes time - perhaps it was faster/easier to move to Germany?

The home-schooled teens that go on to college may lack formal paperwork form a school, but they generally do have some test results, paperwork filed with the county/state, and other items they can put on a transcript in lieu of actual grades.

It sounded like the scientist didn't have the benefit of a structured home education.

Exactly. It's not just that he was home-schooled. It's that he was apparently home-schooled by people who were apparently particularly incompetent at home-schooling.

Yes, the singular goal of all Hutterite parents is to raise their children to be Ph.D scientists. They utterly failed here.