The article is conflating two concepts at Amazon. There is "devplan/focus" and there is performance improvement plan (PIP). You are not fired while on focus. Despite what one of the people indicates in their comments, you cannot be on a PIP and not know. The PIP is an agreed upon plan with a specific output between HR, management and the employee. Firing happens through PIPs, not focus. Though focus is what leads to a PIP.

I agree the lack of transparency on the focus is stupid (especially since it can prevent you from moving), but you don't wake up one day and end up fired. You will be put on a PIP first and be well aware of it (since you have to agree to the plan or take a payout to leave instead).