Firstly, all Grand Jury investigations are secret; its the whole point of the exercise. To suggest that there is something untoward in calling a Grand Jury in the absence of Assange is playing a cheap psychological device to fool the impressionable.

Secondly, Treaty Law is a matter of findings of fact. The primary fact in this case is whether Assange's rights will be impinged by the system of jurisprudence as it applies in Sweden- unless something has drastically changed there in the last five years it will not. We may not call what he is alleged to have done rape but they do- it is representative of the broader issue at stake which is the differences between the two systems involved.
As an officer of the Swedish Judicial system, the prosecutor can lodge arrest warrants without recourse to judicial oversight. Similarly, Swedish Judges can issue their own arrest warrants without being prodded by the police or prosecutors. That's how they practice law. Again, we do it differently but it does not automatically follow that Assange is going to be denied justice because of the differences in these practices.
In this case, the judges need decide nothing more than if Assange will find justice in a Swedish court. Who issued the warrant is immaterial to this central fact.

In case readers are confused by the comment from bren122, it is necessary to point out that, while four grounds of appeal were argued, the Supreme Court said it would consider only one; it said the sole issue is whether a Swedish prosecutor can issue an arrest warrant under the UK Extradition Act.

This is a question of law not fact; it is a matter of interpreting the meaning of ‘judicial authority’ according to long-settled principles of statutory interpretation, but taking into account basic principles of the Framework Decision, as explained by the High Court judges.

Bren122 is right to say it is crucial to see if Assange’s rights are ‘impinged by the Swedish system of jurisprudence’, but wrong to think this is decided by looking at Swedish practice - it would be just as wrong to suppose the test is British practice. This point is explained more fully in the first article, which I should have cited.
The Grand Jury was noted to remind readers how easy it is to get an indictment when a prosecutor has complete command of the process, the accused has no representation, and everything is done in secret. The fact that this might be accepted practice in the US is no answer to the unique peril it poses to Assange when it then becomes the basis for an extradition warrant.