> Obviously it is true in a single threaded system.

But only if you disable interrupts, right? Of course, if you are working that close to the hardware, it makes sense to include "no interrupts" in your definition of "single threaded".

> Note that garbage collection itself can have a switch, not involving disabling interrupts.

I don't understand this. If you get thrashed with interrupts then no switch will remove the overhead of the interrupt handler, so you'd need an upper bound on the time for that handler AND the frequency of hardware interrupts to achieve hard real-time.