I actually spotted the usage in the UI code
Blizzard_Collections/Blizzard_MountCollection.lua:198 first before linking documentation. Note
MountJournal_GetMountInfo() is a shortcut in the file to calling
C_MountJournal.GetMountInfo().
I corrected the entry in WoWProgramming, so it should be accurate now. The API didn't change at any time, so it isn't the matter of being out-of-date. Both sites use a wiki structure and people sometimes make mistakes writing these entries. It's up to other users to correct these as they are encountered.