Translation:It is quite delicious.
In my dictionary (AEDict): 結構 [けっこう] is indicated as a na-adjective does this mean the sentence could be written as:
結構 [けっこう]: 1 (P, adj-na, n, n-adv) splendid, nice, wonderful, delicious, sweet, 2 (P, adj-na) sufficient, fine (in the sense of "I'm fine"), (by implication) no thank you, 3 well enough, OK, tolerable, 4 (P, adv) quite, reasonably, fairly, tolerably, 5 (P, n, arch) construction, architecture
Basically (and originally), "zenzen" means "wholly", "completely", "totally", "absolutely" etc..
Nush is not wrong. if "zenzen" is used in negative, it means "not at all". However, it is not "usually" used in the negative. "zenzen" is used with positive (or non-negative) words.
Some people say these usages ("zenzen" with positive words) are wrong. But there are studies and papers saying this ("zenzen" should be used with negative words) is rather a superstition.
Younger people use more aggressively, like "zenzen sugoi"="quite awesome", "zenzen kirei"="so beautiful" and "zenzen oishii". These kinds of usages are new . (Is it downgraded than original meaning?)
So, won't "zenzen oishii" work as well? I can say it will work in conversation. But, should it be accepted in Duo? I'm not sure.