Hi Beverley,
I believe this article is referring to email templates within your template work library. If you have a client request built into your work flow, the email template within the client request can have these merge fields.
The stand alone email templates currently do not have this feature. We have created several with placeholders for Client Name] etc. that have to be manually changed when we use the template. It still saves us a lot of time and effort. It also allows us to have a consistent response to specific situations. We use this for IRS letters received with a balance due that the client needs to pay.
I believe this article is referring to email templates within your template work library.
This also appears related to the SuiteFiles integration (although possibly not restricted to). Merge fields for regular email templates would be a great feature @Beverley!