"Glyph" comes indeed from sculpting, so it was originally applied only to inscriptions carved in some durable material, like stone, wood or metal.

Nonetheless, nowadays it is a word commonly used when discussing typefaces (i.e. fonts), to designate the shape of a character, as opposed to its meaning. The same character, e.g. the Latin letter "A", can have various glyphs in different typefaces.

Before digital typography, the letters of a typeface were carved in metal by the typeface designer, so the term glyph was appropriate at that time.

Nowadays, it has persisted when used for typefaces drawn on computers, like other typographic terms, e.g. "font" referred to the process of casting molten metal into letter-shaped moulds.

In TFA the word glyph, actually geoglyph, is used in its original sense, not in the typographic sense, as it refers to geometric figures obtained by earth-working, so they are indeed in 3D, sculpted in earth.