It's not hard. The very start of the article says she trusts proper kernel threads more. But a bunch of these languages were designed and written in a time befo

It's not hard. The very start of the article says she trusts proper kernel threads more. But a bunch of these languages were designed and written in a time before threads were much of a thing. So they fake threads in user mode rather than fix the assumptions of the runtime. Now, using kernel threads uncarefully will lead to different problems, which are famously tricky... But what I mean is, it is not "omg kernel threads are hard" that is the primary factor preventing direct access to them. It's that the language runtime has a lot of baggage.