I read them to mean that the one GC thread would stop the world for no more than 10 out every 50 ms, and in addition the concurrent garbage collector could ask

I read them to mean that the one GC thread would stop the world for no more than 10 out every 50 ms, and in addition the concurrent garbage collector could ask any mutator thread to pause and do no more than 1 ms worth of GC work during any 40 ms non-STW period. So in terms of real time, your deadlines could be no less than 11 ms.