[I’m a PM on EventBridge] This is correct. Tim Bray, who built CloudWatch Events (which later became Amazon EventBridge) participated in the early CloudEvents spec, but as Erik said, CloudWatch Events launched before CloudEvents spec was finalized.
From the EventBridge perspective, we like CloudEvents and I expect that in future we will look to add support for it as an alternative to our existing envelope.
Changing the CW implementation which is so tightly coupled to many other AWS service might be overly tricky for AWS to accomplish over a short period of time. Hope it happens anyway
[edit: more simply MS & GCP said were insufficiently flexible for the Gorilla and AWS wasn't willing to delay shipping]