Even if you grief a lot, if you're dealing with it yourself you will not get a diagnosis.
If grief makes you isolate yourself, stop working, start abusing alcohol or drugs, stop you from caring for you children, etc. Then there's a very real chance that this behavior could lead to more problems, making it in turn harder to regain positive mental health. In that case the person should get a diagnosis, which would then unlock treatment and therapy for them.
> The American Psychiatric Association describes “disordered grief,” also known as “prolonged grief,” as a loss that occurred at least one measly year ago for adults
Even though that might be true, it's an excellent example of why you can't just take the written word at face value if you're not well versed in the subject. In the mental health profession it's understood that everybody is going to be different from the norm. Only the treating psychologist can put the clients problem in the proper context to determine if a year is enough or not.