There's nothing in the thread to suggest he's trying to conceal anything meaningful in terms of the content of the correspondence. Presumably the "how does I delete sexy snapchats to the Saudis" thread was done more discretely.
The reason Exchange does not have the functionality to redact email address is that people who need to redact information usually prefer to do it by hand, so it would be a waste of Microsoft's time to build a utility to do it.
But to think that an Exchange DB can't be altered... I mean, if you have root on a machine you can do anything you want to the information on it. And yes, the legal world knows that.
As a business person, nobody would [buy/use] exchange if it was not reasonably secure from an audit trail perspective. The intergrity of the communications is required for many business's who have record retention policy and what not.
Think about an analogy for a bank's accounting system that allowed audit trails to be compromised. Its a huge problem for the purchasing people and the managerial layer that has to sign off on sarbox etc.
Even if you are using journalling, an administrator can open the journal mailbox and edit messages. If you want/need a reasonably secure audit trail, you need a 3rd party product in addition to Exchange.
[1] https://www.msoutlook.info/question/edit-message-and-subject
Technological audit trails are trustworthy only to the extent that the admins running them are trusted.