"Where is paradise?"
Translation:Peponi ni wapi?
I'd agree with you, but I've also seen ni used a lot with locatives. I'm not sure how correct it's regarded as. I know that, at least in tenses other than present, where you have to use a form of the verb kuwa, the -po, -ko, -mo become optional if it's clear.
Eg. "I was here" can be:
Nilikuwepo hapa. (some speakers)
Nilikuwa hapa. ("Hapa" makes it clear enough.)
So bringing that to the present tense, where kuwa is replaced by ni/si and kuwa[LOCATIVE] is replaced by [SUBJECT.PREFIX]-[LOCATIVE], you can see that using "ni" here is analogous to using the simple kuwa without the locative in other tenses.