No such app as "Mac OS X Installer" -- why does this page reference it?

Because that's what you'll get when you download Mavericks, an installer titled "Install OS X Mavericks" as stated in the support article.

Regards.

Actually what the poorly written article says is

"

Before you're able to create a bootable OS X installer, you'll need to do the following first:

Download the OS X Installer app from the Mac App Store.

which your reply makes clearer that you already have this as a result of downloading a new version of OS X but not everybody understands that....

Can you still get the Mavericks Installation app? All I see is Yosemite installation bits. (I need to test with OS X Mavericks still)

Only if you previously downloaded Mavericks before. If you did, check the App Store Purchases tab.