Not necessarily - the examples listed above are
specific instances where the Entity initialization code checks for custom reactions. While you could use that to add types of metals, it would not be possible to add types of cloth.
For those interested, here is a
full list of places where custom reactions can grant materials to civilizations, as of version 0.34.11 (0.40.xx has not yet been checked, but they are likely to be the same):
* Nearby minerals (based on geology near sites) with [SOIL]/[SOIL_SAND]/[SOIL_OCEAN] and [MATERIAL_REACTION_PRODUCT:FIRED_MAT:something] will add said soil material as a type of Clay available for trade. This also applies to the site finder.
* Creature materials (based on nearby wildlife) with [MATERIAL_REACTION_PRODUCT:some_mat:something], such that
permitted reaction contains the token [PRODUCT:any_probability:any_number:SKIN_TANNED:NONE:GET_MATERIAL_FROM_REAGENT:any_reagent:some_mat], will add "something" as a type of Leather available foe trade (in case the leather material itself doesn't have the [LEATHER] token).
* Creature materials (based on nearby wildlife) with [MATERIAL_REACTION_PRODUCT:CHEESE_MAT:something] will add "something" as a type of Cheese available for trade.
* Custom reactions which produce BAR items made of specific metals (inorganic materials with [IS_METAL] but not [SPECIAL]/[DEEP_SURFACE]/[DEEP_SPECIAL]) will add that material as a type of Metal available for trade and general use (i.e. they'll sell bars of it
and use it to make finished goods)
* Custom reactions which produce BAR items made of specific non-INORGANIC materials (i.e. Coke and Pearlash) will add those specific
items as Miscellaneous Processed Wood items available for trade (under the "Other" section).
* Custom reactions which produce POWDER_MISC items of any specific material (i.e. Gypsum Plaster) will add those specific materials as types of Powders available for trade (along with plant mill products).
For a full list of everything an entity can use for trade, see
here (the source of the above list, again only confirmed for version 0.34.11).