I tried to find related code with iam and msk in the file names and failed. To get authenticated with MSK, need to get proper signed and pass to the MSK endpoints. Maybe AWS SDK team are just too busy or not get enough customer ask(escalations?) for using C++ to access MSK cluster with IAM only auth.
Having a SDK and supporting IAM auth for MSK are different stories though. I mean, for supporting IAM auth for MAK for Java, AWS has a separate library instead of putting the code in their Java AWS SDK: https://github.com/aws/aws-msk-iam-auth.