Is there a source for this? I'm genuinely curious. The "AWS started as Amazon's extra capacity" is such a compelling story.
I believe SQS was the first publicly available service. EC2 was obviously the tipping point and that was a couple of years later.
SQS as the first publicly available service sounds weird to me but I don't have any sources to cite otherwise.
Edit: Wikipedia says SQS was the first service https://en.wikipedia.org/wiki/Amazon_Web_Services#History