They are all different Garamond typefaces.
While the bold, italic, display, caption, etc. versions of each are the different fonts.
"A font is an assortment of one size and face of type."
In other words, most faces came in multiple sizes, and each face-size pairing (that you could buy) was called a font. A.T.F. Goudy Bold, to use an example from the book, came in the following sizes: 6, 8, 10, 12, 14, 18, 24, 30, 36, 42, 48, 60, 72, and 96 (all units given in typographer's points). Complicating the matter was that each font had a slight variation of the face design to accommodate the way that ink behaved on the type differently and was interpreted by the human eye differently at different sizes.
Edited to add: But none of this changes the fact that Adobe Garamond and Garamond 3 and ITC Garamond are not the same typeface. If you wrote the original article, you should probably update it to fix this mistake before readers start getting the wrong idea.