That's not how I understood it. It says they're doing a hybrid STW and concurrent GC.
(a) A mutator may be stopped for up to 10 ms of every 50 ms by the STW collector.
(b) If more collection is needed, it will transition to a concurrent collector for the remaining 40 ms. Concurrent may pause stop a mutator for up to an additional 1 ms.
Therefore the worst case is 10 + 1 = 11 ms, not 1 ms.
(a) A mutator may be stopped for up to 10 ms of every 50 ms by the STW collector.
(b) If more collection is needed, it will transition to a concurrent collector for the remaining 40 ms. Concurrent may pause stop a mutator for up to an additional 1 ms.
Therefore the worst case is 10 + 1 = 11 ms, not 1 ms.