I worked at HubSpot in engineering and I never once heard the term "graduation" in reference to a firing. Usually we just got an email saying, "HubSpot and John have decided to go separate ways" or something to that effect. If someone left by their own choice, usually they would send a "this is my last week" email and we would all go for beers on the last day. The term "graduation" was rarely used in either case. I only remember it being used in the context where an executive would say something like, "John did great work for us, he's now going to be a VP at a new startup, we want to think of this as a graduation."