The links you found are, no doubt, from old posts. The structure for Guides has changed since then, but it would be impossible to find every link to obsolete pages.
What your screen shot shows are the default tabs. Until you enable the Cash Accounts tab and create an account, you have no need for exchange rates, so the option does not appear. Set up a cash account:
As soon as you enter any account or transaction that permits a different currency, the Exchange Rates option will appear under Settings.
This behavior is a basic characteristic of Manager: almost nothing shows unless needed.