As @Calinou said there, it's up to the OS how it allocates threads to cores. So if you spawn a thread it might end up on another core or it might end up given to the same core if the CPU in question features simultaneous multi-threading. Or even in queue on the same thread if all the available threads are already under active allocation.
But yes, i guess you could make the argument for this being multiprocessing, though I'd argue that multiprocessing is when you actually spawn multiple processes(like how say the chrome web-browser might show up as multiple chrome.exes in the windows task managers processes list when you have multitude of tabs open).
I'm not 100% sure but I don't think that's happening when you spawn a thread with godot. Haven't actually payed attention to that when using threads in godot though so I could well be wrong.
