The error is thrown by your AWS because it cannot assume Snowflake, and is definitely related to the AWS permissions.
Your trust policy is correct as per https://docs.snowflake.com/en/user-guide/data-load-snowpipe-auto-s3#step-5-grant-the-iam-user-permissions-to-access-bucket-objects

I could not think of other reason for the error.
Were you able to get this work? If you still need help, let me know. I can assist through a Snowflake support case.