To assume a role with OIDC you'd need to do it from the context of a specific CircleCI job run - getting access to the secrets of a particular CircleCI account alone would not be enough to authenticate to AWS (unlike when you use IAM user credentials).

Even if the attacker had access to env vars from running jobs (which includes the signed token needed to do an OIDC role assumption), those tokens have a short expiry time, and even if an attacker stole that token and performed a role assumption then that session can only be valid for a maximum of 12 hours in AWS, and then you know the attacker is out of your account.

It just significantly reduces, practically nullifies, anything that an attacker can do in your AWS account.