That's true to a fair degree. If you're running your AWS service using a relational database in AWS, they'll have your guts for garters. However, that's because of the sheer level of scaling that services have to operate at. Consider the number of requests per second that they handle.
Scaling gets fiendishly complicated under those circumstances.
That's not to say that it's impossible to do. Facebook ably achieves it. It's just that the level of expertise that would be required across so many services is significant. AWS has hundreds of services, each of which would need to be hiring highly skilled DBAs to handle the sharding etc. etc. etc necessary to scale. It's easier to just point people at DynamoDB where they've effectively handled all those needs for you, you just have to put a bit more logic in your application side, which also has the neat property of scaling horizontally more easily.
That's not to say that it's impossible to do. Facebook ably achieves it. It's just that the level of expertise that would be required across so many services is significant. AWS has hundreds of services, each of which would need to be hiring highly skilled DBAs to handle the sharding etc. etc. etc necessary to scale. It's easier to just point people at DynamoDB where they've effectively handled all those needs for you, you just have to put a bit more logic in your application side, which also has the neat property of scaling horizontally more easily.