I take that to mean, that despite the context and the second half of the sentence, the "prototype" mentioned is some kind of Perl 6 language prototype, and not any kind of Parrot threading prototype.

That leaves just the existing PPD, which hasn't changed since it was drafted; is based upon the Perl 5 iThreads model; was written by people who had never used that model; makes no concessions to the obvious failures of the only existing implementation of that model; seeks to perpectuate all the mal-features of that model; and will have to be tacked-on to the Parrot single tasking architecture after the fact just as the existing implementation was.

Oh dear.