An ambassador is a diplomat and not all diplomats are ambassadors, yes. But the distinction has very little to do with length of appointment.

An ambassador is the head of a diplomatic mission who is the representative of his/her head of state (President, Prime Minister, King etc.). Ambassadors may have spent many years working for the ministry of foreign affairs (career ambassadors) or they may just be friends or supporters of the head of state (political appointees).

However, ambassador can also be simply a rank or title. Ambassadors-at-large are also diplomats, but not head of a diplomatic mission. Former ambassadors may keep the title even after their assignment is complete.

Because an ambassador is the representative of the head of state, their assignment may end when the head of state changes - this is apparent in the US when a new president is elected, when political appointee ambassadors are largely replaced. Their appointment may end up being very short.

A political appointee ambassador, almost by definition, has not been/will not be a diplomat for long. A career ambassador has very likely been a diplomat for many years, rising the ranks. Still, another diplomat may have served just as long or longer, but didn't get the same promotions so isn't an ambassador.