I guess it depends on how you define "search" and "everything".
According to what I've read Spotlight can only search content of files that have predefined plugins that define the contents of a "kind" of file. So if you want to search for something that does not have a "plugin" then you're out of luck. Many kinds of files have these plugins, like your Pages example, and some don't.
So claiming Spotlight can search the contents of all files is not accurate or true.
For a very simple example try adding your username (barney-15e) to an XML plist file (a simple ASCII text file) - Spotlight won't find it no matter how many times you rebuild the index. If you still want to claim that it can search everything then I guess that you can call that a fact if you wish - an "alternative fact" is more like it. If you want to say it can search the content of "some" files, then that is a real fact. But don't advertise it with an ability it doesn't have or will ever have - the overhead would simply be too large.