I've been looking (way too) deeply into this, and technically, you are correct about this to the best of my research.
But there are a few ways out. These truths do not mean this motion has a chance.
For one thing, the police don't have to release truthful information about their investigation. Maybe they have a GJ witness who saw a man with a gun talking to Amber as she climbed into his car without screaming, for fear of getting shot. It's a baseless theory, but my point is not its merit, but the possibility that something is going on that LE does not yet want the defense to know, at least not until they decide to use that piece of evidence. If the hypothetical evidence they are trying to distract from about a gun pans out as exculpating or both inculpating and going to be used, the defense has to learn about it eventually. But LE could decide the evidence does not exculpate or help them enough to bring it out. So they could want to keep it under wraps until they have to bring it out to use it or to play fair (share exculpating evidence).
Or maybe Suffolk County LE believes what they said about the firearms and the GB-4.
No matter what the truth is, they can easily flip and say, "Never mind, we are still investigating and we need these guns in our custody or with our partners in Nassau."
It does not seem, from lawyers blogs or articles on the subject, that it is going to ever be a winning motion to demand something back that is irrelevant to the investigation, and nearly impossible to get something back that is relevant. So, one thing LE could do if they are still looking into other crimes and haven't charged Rex with them yet, or obtained a warrant, is to simply tell the judge, we're not done with them yet.
But, what if Suffolk County wants to double down on saying that none of the guns were used in the GB-4. Because that is the truth, yet they think a firearm is relevant in another investigation they have not yet admitted to working on? All they have to do is get a new warrant. Don't quite have probable cause yet? Go to plan A, lie.
So LE has all the power to and all of the evidence-seized precedents in court in their favor. All they have to do is 1) say the current investigation is ongoing and they do need the firearms - or 2) get a new warrant.
To help out a little on this. IANAL, but i'm pretty sure that, once LE has arrested a suspect, courts have charged him and they move towards trial, LE cannot lie about evidence, etc.
They can lie to deceive or trick a suspect, but no more once he's arrested.
What they say now about evidence is supposed to be factual. In the case of statements about whether any guns from his collection were used in murders, the DA should be making a general statement. Actually, I was surprised to hear them say that when they hadn't finished examining all the evidence from their searches and when they're still investigating possible links between him and other victims.