Hi IT_Nick , no worries on the age of the blog post - I am happy to help!
So, the blog post just contains example of the final state, you can set the mode to testing to start things off and do some validations before switching the mode to enforce -
Some helpful resources -
https://learn.microsoft.com/en-us/purview/enhancing-mail-flow-with-mta-sts?view=o365-worldwide#option-1-recommended-azure-static-web-app
RFC 8461: SMTP MTA Strict Transport Security (MTA-STS) (rfc-editor.org)
- details on mode, don't use none unless you are disabling MTA-STS
o "mode": One of "enforce", "testing", or "none", indicating the
expected behavior of a Sending MTA in the case of a policy
validation failure. See Section 5, "Policy Application", for more
details about the three modes.