The Smalltalk mechanism being referenced was the class hierarchy browser. It was invented in 1976 by Larry Tesler (see Alan Kay "The Early History of Smalltalk" under "Smalltalk-76"). The browser used a fixed 4-list view to locate source code for methods. The first list was for categories. Within each category you could select a class in the second list. Once a class was selected, a method protocol could be selected from the third list. Finally, once a method protocol was selected, a method could be selected from the fourth list. Upon selecting a method, the source code for the method appeared in the text editor below the 4 lists. This is an example of a special-purpose fixed-depth miller column.
From my reading of the wikipedia article, it seems that Miller columns were more general and had an arbitrary number of columns. It's believable that Miller had no knowledge of the Smalltalk browser - the browser wasn't made generally available until the public release of Smalltalk-80 in 1983. Before then, it was only used by people associated with Xerox and by a handful of companies that were asked to port Smalltalk-80 to their own hardware. (See Smalltalk - Bits of History and Words of Advice). Screenshots of a class hierarchy browser did appear in the 1981 issue of Byte magazine (page 124 onward) but again, this was after the 1980 invention of Miller columns.
It makes sense, then, that the more general mechanism was named after Miller with people afterwards noticing similarities to the Smalltalk browser.