If you look up the word ‘biscuit’ in the Oxford online dictionary, you'll see this
1 British A small baked unleavened cake, typically crisp, flat, and sweet:
[as modifier] ‘a chocolate biscuit’
More example sentences Synonyms
If a chocolate biscuit is a biscuit, albeit made with chocolate, shouldn't ‘chocolate’ be the modifier? This questions whether I really know what a modifier (or a chocolate biscuit) is. Or it's just an error.
Comparison with ‘oil’
‘400 birds were coated with oil which spilled out from an opened valve’
[as modifier] ‘the oil industry’
This is in agreement with the common definition of grammatical modifier. I don't think ‘a chocolate biscuit’ is a modifier as a whole: it wouldn't be an example with ‘biscuit’ used as a modifier (and it wouldn't make much sense).