The documents you have linked to explain the difference between Sitecore Email Cloud or Custom SMTP. The answer from @Vlad also describes how to switch between the providers in Sitecore 9.

If you are trying to configure the mail server in order to send emails using the default `Email Workflow Action` created using `/sitecore/templates/System/Workflow/Email action` then you do not need to worry about the Sitecore Email Cloud or Custom SMTP settings, and you should specify the STMP details in the `Mail server` field of the Workflow Action.

[![Workflow Email SMTP][1]][1]

Those settings are ore relevant for Email Experience Manager ans anything which uses this module to send (mainly bulk) email.

The default Workflow Email implementation is `Sitecore.Workflows.Simple.EmailAction` in `Sitecore.Kernel.dll` and this sends email using the [`SmtpClient` class][2]. The implementation is very crude IMO, and you would better providing your own implementation allowing the use of an empty `SmtpClient` constructor to [use the configuration settings from `web.config`][3]. 

Alternatively, you can use the `Sitecore.MainUtil.SendMail(MailMessage message)` helper which will allow you to set and use the `MailMessage message`,  `MailServerUserName`, `MailServerPassword` and `MailServerPort` settings from the default Sitecore config. Both these are idea for simple system messages an and notifications (EXM is better for more complex emails and campaigns).


  [1]: https://i.sstatic.net/IsIk3.png
  [2]: https://msdn.microsoft.com/en-us/library/system.net.mail.smtpclient(v=vs.110).aspx
  [3]: https://stackoverflow.com/a/11845555/661447