Autism seems more like a symptom than a condition.

"Stomach ache" is not a spectrum disorder, even though is comes in many severities. It's a symptom of dozens of different medical conditions.

I suspect "autism" is similar.

Autism isn't a symptom because Autism Spectrum Disorder isn't a singular "thing" it is a combination of features that manifest in various "symptoms" - eye contact avoidance (or the opposite), sensory processing (over- or under-stimulating), restrictive interests (singular focus), etc.

A stomach ache is a single manifestation of something happening. A stomach ache can have varying degrees and reasons behind it. The stomach ache is the signal. The disorder that causes it could be psychological (GAD) or external (someone punched you, and the flesh is bruised) or internal (someone fed you a weeks old egg salad sandwich).

In autism the 'symptom' could be model train enthusiasm or being nonverbal. There are a lot of symptoms the fit under the umbrella of the disorder.