Hello @Les
Yes, the document Turn-off-directory-synchronization is correct to convert synced users to cloud only users as it turns off the directory sync from on-prem to Entra and removes all on-premises properties which makes the user cloud only and then the user can be managed in cloud i.e. Entra. This document has the PowerShell commands documented to perform this change by using MS Graph PowerShell module.
However, do note that the commands provided in the document would turn off the sync for your entire synced users/groups and will convert all those synced users/groups to cloud only. This cannot be utilized to target any specific user or group as this is an organization wide change.
If you have any further query, please do let us know.