[chrisseaton]

I'm not sure what you think is the important difference between these implementations?

'Dynamic binding' is an abstract concept. The abstract concept is a global variable with a stack of values. How you implement it and whether you share the call-stack instead of a separate stack is is up to you. It's still the same dynamic scoping.

[tlb]

Only one of them works with multiple threads. Dynamic scoping based on save-and-restore of symbol value slots is completely incompatible with multithreading. That's why Emacs still doesn't have threads.