It's true that the "ez" is not a ligature in its original sense, but "ex" and "os" are definitely ligatures. Technically, if "ez" is defined in the font as a single glyph, then it's a ligature too.
Kerning affects only the spacing.
The main point here is that the text contains only "e" and "x", not a special character for the combination "ex". And the font has the information to know that whenever "e" and "x" are together, it shouldn't draw the normal shapes (top line), but instead use a special glyph that contains the "ex" combination.
So, if a book contains the word "office", do the Kobos, with any font, display an "ffi" ligature?
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