Anonymization in the data reselling industry is often some form of md5(lower($email)). It's a joke. They even do that for extremely small search spaces like phone numbers. It's still provided at the individual user-level and even if the anonymization is done in a way that's irreversible, you only need to know a single event for a given person and you now have their entire history.

For example, there's a popular email client that scrapes people's inboxes and sells their purchase history to anyone willing to pay. That purchase history is provided on an individual email level and is "anonymized". But if you know your target has this email client installed and you know a single purchase (e.g. a coworker saying "Oh, I bought this awesome coffee maker on Amazon last night!") you can now access their entire individual purchase history backward and forward.