I see it too, it seems to be extra. Perhaps Toady just let it slip? something that he wanted to add but couldn't yet? because:
[USE_MATERIAL_TEMPLATE:STRUCTURAL:STRUCTURAL_PLANT_TEMPLATE]
[MATERIAL_VALUE:2]
[MATERIAL_REACTION_PRODUCT:DRINK_MAT:LOCAL_PLANT_MAT:DRINK]
[MATERIAL_REACTION_PRODUCT:SEED_MAT:LOCAL_PLANT_MAT:SEED]
Here the material is marked as the structural material (this could be below the edible tags which come next). In general, you can use LOCAL_PLANT_MAT|<token>, PLANT_MAT|<plant>|<token>, CREATURE_MAT|<creature>|<token> or INORGANIC|IRON (though the game might hiccup for a while specifically on plants that aren't structurally plants).
[BASIC_MAT:LOCAL_PLANT_MAT:STRUCTURAL]
We also modify it a bit to make the plant edible. Any token material can be used here to modify the material that was created from the template.
[EDIBLE_VERMIN]
[EDIBLE_RAW]
[EDIBLE_COOKED]
[USE_MATERIAL_TEMPLATE:MUSHROOM:STRUCTURAL_PLANT_TEMPLATE]
[EDIBLE_VERMIN]
[EDIBLE_RAW]
[EDIBLE_COOKED]
STRUCTURAL is defined as normal, then the [BASIC_MAT:STRUCTURAL] call is made, then the edible tags are tossed in. Wouldn't DF just ignore those since now there is a break in sequence of tags? And then the MUSHROOM material is defined but nothing is done with it, it's just an edible material (which I think may be the only edible part, just going by the structure of the raws so far) that is sitting there doing nothing and cannot be reached.
now other plants that can be eaten as is (which plump helmets are) have:
[USE_MATERIAL_TEMPLATE:STRUCTURAL:STRUCTURAL_PLANT_TEMPLATE]
[MATERIAL_VALUE:1]
[EDIBLE_VERMIN]
[EDIBLE_RAW]
[EDIBLE_COOKED]
[MATERIAL_REACTION_PRODUCT:DRINK_MAT:LOCAL_PLANT_MAT:DRINK]
[MATERIAL_REACTION_PRODUCT:SEED_MAT:LOCAL_PLANT_MAT:SEED]
--- I am also looking over the fact that bloated tubers don't have a seed material which makes me overly concerned on how this will work in custom reactions that may call upon SEED_MAT and find nothing.
Snippets taken from "40.04" vanilla (no modifications made on my end)