The part about detecting stack overflows probably refers to implementation strategy in some scheme implementations (IIRC chicken scheme does this) which involve

The part about detecting stack overflows probably refers to implementation strategy in some scheme implementations (IIRC chicken scheme does this) which involve using the platform/machine stack as effectively hardware-assisted nursery generation for generational copying GC, when it gets full (usually well before true overflow) the minor GC cycle gets done and the stack gets completely emptied. In these designs the "stack" is usually used for all allocations, not only for activation records.