[plt-scheme] Thread support

Greetings,

In the manual for MzScheme 299.26 it states for threads:

Chapter 7:

"MzScheme supports multiple threads of control within a program.
Threads are implemented for all operating systems, even when the
operating system does not provide primitive thread support."

And in the Introduction in Chapter 1:

Pre-emptive threads (see Chapter 7).

The chapter 7 note makes me think cooperative threads are used at least
in some cases (on those implementations where the OS doesn't support
native threads) but possibly not in all.  The note in chapter 1 makes me think
true OS pre-emptive threads are supported where available.

I find this description ambiguous and it has caused me many, many hours
of wasted work in prior releases.  I understand MzScheme 103 now but am
unclear about 299.xx.  Can't the docs be updated to be clear?

Are true (pre-emptive,OS level) threads ever supported by MzScheme?

Are all implementations done with portable, internally cooperative, single
OS thread implementations?

Thanks.

Blake McBride