The full name is "Autistic Spectrum Disorder", so the name hints that there's a range of stuff happening.

To be autistic someone has to have problems with social communication, problems making or maintaining friendships, and fixed and repetitive interests.

Some people also have other stuff on top. These things are common with autism, but are not needed for the dx.

Alexithymia (the ability to recognise emotion in yourself and others) is one example. It's far more common in autistic people, but you don't need it to be autistic. Between 50% and 55% of autistic people have alexythimia. Sensory sensitivities are another. There are a range of these things that are more common in autistic people, but aren't needed for the dx.

And there's a lot of co-morbidity too. People with autism are more likely to have depression or anxiety or OCD. These aren't part of autism, but it's complicated to untangle what's going on. Is someone social isolated because they're depressed, or autistic, or is it a bit of both?

When you start looking at these other things it makes sense that autism might be an umbrella diagnosis.