Your misunderstanding that comment. It was built "in the same datacenters by the same IT engineering team" but it was always it's own thing. "AWS services were quickly adopted by many teams" after it was built and a commercial product.
Sure, the intent was Amazon IT would eventually move to that system. But, they could let other companies work out the issues first vs. risking their main website. Just consider the risk for them if AWS goes down and take their own site outline while they are trying to sell high up time. That's simply a risk they had no reason to take.
PS: Remember, well executed traditional large scale infrastructure has higher up-time than AWS because AWS adds complexity.
Sure, the intent was Amazon IT would eventually move to that system. But, they could let other companies work out the issues first vs. risking their main website. Just consider the risk for them if AWS goes down and take their own site outline while they are trying to sell high up time. That's simply a risk they had no reason to take.
PS: Remember, well executed traditional large scale infrastructure has higher up-time than AWS because AWS adds complexity.