Multitasking and Multithreaded are sometimes confused in modern terms. I think that the AmigaOS was/is a multitasking OS but I don't think that its multithreaded which causes issues like this. It really comes down to Exec and if I had to guess I would say that its not multithreaded. I don't know about 2.x/3.x...
The reality is somewhat different. Amiga always had (and still has) "tasks" and "processes". While a Unix process can´t easyly access another processes memory and have a lot of overhead, there was a need for some sort of more lightweight multitasking. Hence the invention of threads. So Amigas tasks resemble threads in many ways.
