Yes, because that's what happened. "The questions were prepared by prosecutors in Sweden, where an arrest warrant for Mr. Assange was issued in 2010, but were posed by a prosecutor from Ecuador under an agreement the two countries made in August." - https://www.nytimes.com/2016/11/15/world/europe/assange-wiki...
I think a key part of a police interview is asking new questions based on responses and having an actual conversation, which the Swedish prosecutors couldn't do.
Your original comment made it sound like they had no input in the questions being put to Assange. Merely "present" to observe Ecuador. This article suggests Sweden was involved and I don't see anything suggesting the procedure (under "an agreement the two countries made") was problematic. Who asks the questions seems to just be a technicality here.
"Isgren is allowed to ask Assange to clarify his answers, but not to put additional questions, and will receive a written transcript of the exchanges from Ecuador after the interview has concluded." - https://www.theguardian.com/media/2016/nov/14/julian-assange...
Problematic for whom? You seem more concerned about this than the Swedish prosecutors. It seems this was the procedure they agreed to. Whether it's normal or not I don't know. But they haven't made a fuss about it. And I think they would have if it was a big concern.
They could ask for a second interview and present follow up questions.
But this raise a primary question. Is the distinction of being able to instantly make follow up questions of such importance that it will swing the binary choice of charging Assange of a crime or dropping the case?
The primary cause for a guilty and not guilty verdict should not be the quality of the environment for a police interview. Not for a such serious allegation like this.
I think a key part of a police interview is asking new questions based on responses and having an actual conversation, which the Swedish prosecutors couldn't do.