It depends on how you set up your managed metadata, since both ways are possible. Using enterprise keywords enables a tagging system where users can select existing keywords or create new ones. It's similar to tagging a question here, where you select existing tags or, if you have 300 reps, create new ones if they are needed.
Keywords have no hierarchy, so I guess the article referred to managed terms, where only user with the right permission can edit managed terms.
If you want to allow the user to set up multiple values is entirely up to you, since it's a field setting called "allow multiple values", which you control.
Managed terms, enterprise keywords, and the term store
Terms can be divided into two types:
Managed terms, which are usually pre-defined, can only be created by users with the appropriate permissions, and are often organized into a hierarchy.
Enterprise keywords, which are simply words or phrases that have been added to SharePoint Server 2010 items. All enterprise keywords are part of a single, non-hierarchical term set called the keyword set.
Reference and more to read: Managed metadata overview (SharePoint Server 2010)
Edit
To prevent users from adding their own categories follow these steps:
In your wiki site, go to Site Actions > View all site content
Click on the Pages-library
Click the library tab in the ribbon
In the Settings pane, click the Library Settings
In the Columns section find and click the Wiki Categories column
At the very bottom - find the Allow fill-in option and select No.
Click OK to save your changes
