κουλουράκι would be mainly translated as shortbread and in this sentence it is not an accepted alternative. Κουλουράκια and μπισκότα are two different words for Greeks and they are not considered synonyms. It's like a τυρόπιτα and a πεϊνιρλί... :-)
Shouldn't "κουλουράκια" be acceptable as well?
I believe there is an error. Probably in the help-box.
Thank you for your input, it has now been corrected ;)