Why would they consider themselves to be on the spectrum then? I mean technically the whole thing could be a spectrum of not austic to autistic. But it seems odd to have the spectrum not end at either point, the diagnosis or the one capturing all people.

This is blatantly wrong. A person who is "in the spectrum" is a person who is autistic, regardless of whether they had a formal diagnosis or not. They're just different ways of saying the same thing.

"On the spectrum" is both a soft term of art and very often a euphemism for either never seeking professional diagnosis or not being able to convince professionals that you warrant a diagnosis. It's also used to describe clusters of distressful symptoms which have not received a classification.

The difference between the population of those who have received a diagnosis and those who are on the spectrum is night and day, and it's often the line which separates whether you'll have any reasonable chance at life. The OP suspects his coworker has received such a diagnosis.

So when I hear OP say that his coworker may have a diagnosis, that absolutely changes the picture in my head. It's up to you to decide whether professional medical diagnosis is a signal to you.

Have a watch on this: https://infosec.exchange/@cakeisnotalie/111339054583410032

Next time, please be more empathetic to the people who have not been diagnosed for whatever reason. "On the spectrum" means autistic. Whatever other interpretation you might want to give it is on yourself.

If you have cancer, I would recommend that you do everything possible to convince your doctor that you have cancer. There is no adequate at-home test for autism. Clinical assessment is the gold standard.