the best link for understanding what happened is actually the court case filing not the media reports. https://www.justice.gov/usao-wdwa/press-release/file/1188626...
so this isn't a case of s3 bucket being public/wide open, its a case of a waf iam permissions being overly broad if I'm parsing the filing correctly. Its unclear how the waf product was hacked/bypassed and its credentials obtained.
wrt to custodian in this equation, its not really related afaics, custodian has lots of filters to help determine stuff like is my ec2 or anything with iam role (lambda, etc) overly permissive wrt to permissions (check-permissions filter). it also has the ability to filter individual statements and access on any resource (s3, lambda, etc there are many) with an embedded iam policy on a fine grained basis (allow y accounts but not x accounts) to protect against account level access (cross-account filter). And the ability on ec2 via guard duty alerts to auto remediate (suspend, memory snapshot, yank role, volume snapshot). its used by lots of users/enterprises across the governance, security, cost-optimization domains because its flexible and supports many clouds.