It isn't open to interpretation at all. Habeas corpus was denied on the grounds that habeas corpus didn't apply since Hamdi was an enemy combatant, not that habeas corpus had been suspended.
Congress hadn't suspended the right of habeas corpus, so habeas corpus can't be denied to a prisoner even if the Executive supports denying it. The decision said the Executive department could not deny Hamdi the right to be heard in court (i.e., habeas corpus) under the due process aspect of the Fifth Amendment.
Nobody had suspended habeas corpus, and Constitutionally there were no grounds to, so the question of who can suspend it was not an issue before the court.
Taney had also said in his Ex Parte Merriman order that due process was being violated by Lincoln.
Yes he did. But the entire Supreme Court did not.
There have been long battles back and forth on FreeRepublic about whether Chief Justice John Marshall's following statement from his majority opinion in Ex Parte Bollman and Swartwout was dicta or not...
The answer to that should be easy. If the issue commented on is not a matter before the court at that time then the comments are made in dicta. Since habeas corpus had not been suspended it was not a matter for the court to rule on. The matter was whether the Supreme Court could issue a writ of habeas corpus.
From Hamdi's lawyer's opening statement:
ORAL ARGUMENT OF FRANK W. DUNHAM, JR. ON BEHALF OF PEITIONERS
MR. DUNHAM: Mr. Chief Justice, and may it please the Court:
Petitioner Hamdi is a citizen who has been held over two years in the United States with no opportunity to be heard as to the facts on which his detention is based. Mr. Hamdi makes two claims. First, the Fourth Circuit wrongly prevented Hamdi in this habeas proceeding from being heard as to the facts of the case on grounds that allowing him to be heard would interfere with executive power.
Second, that the Fourth Circuit erred in finding even on the one-sided record that's before this Court that his detention is authorized by law. The historical core of habeas corpus is to challenge extrajudicial executive detention. It cannot be a violation of the separation of powers for an Article III court to perform its judicial function of inquiry into long-term, indefinite detention of a citizen in a habeas corpus proceeding.
Quoting from INS v. St. Cyr, at its historical core, the writ of habeas corpus has served as a means of reviewing the legality of executive detention and it is in that context that its protections have been strongest.
Now look at the excerpt from the Supreme Court decision:
"it would turn our system of checks and balances on its head to suggest that a citizen could not make his way to court with a challenge to the factual basis for his detention by his government, simply because the Executive opposes making available such a challenge. Absent suspension of the writ by Congress, a citizen detained as an enemy combatant is entitled to this process."