I am a developer. I tested this on multiple occasions and can confirm that the Threshold system is flawed majorly. First of all, even when the option is switched off, it gets considered when showing product stock. Ex: I had previously saved my OOST = 10 and even if my option in atum settings is disabled, the "stock" of product will be displayed Stock-OOST. I did not check if it is the same value available on front-end.
Secondly, on my setup when in Product Data tables I try to save an inventory (Multiple-Inventory), if my input is below OOST, the stock becomes "-1" on update.
I am not sure if it is the correct behavior, certainly doesn't appear to be to me.
I have totally ignored this option for now, and I do not intend to use it either - it adds too many complications. However, I thought I shall report the issue.. it might help others.