I seem to remember reading somewhere that, even at his worst, Linus limited his toxicity to professional-programmer kernel contributors (i.e. people who were employed by linux distros or hardware companies to contribute to Linux). Can anyone else remember/confirm this?

Regardless, to a newbie potential kernel contributor, that high level of toxicity can be intimidating, and the professional-programmers-only aspect is non-obvious, so it's easy to see why this would discourage hobbyists/free-time programmers from contributing.