Stack is an overloaded word, in my experience they are just referring to stack as a "Architecture Tier" level. Also from reading the quote below they are heavily relying on Linux to multitask, which implements a full preemptive multitasking environment.
The nice thing about the Android design is one app's VM is isolated from the other so instability in one VM will not effect the other.
Android relies on Linux version 2.6 for core system services such as security, memory management, process management, network stack, and driver model. The kernel also acts as an abstraction layer between the hardware and the rest of the software stack. (Full link)