You seem to be missing the point. If the investigators DO have knowledge of what happened in 5A, why on earth would they "allude" or draw any attention to it?
If they have information about what they think took place in 5A, there's no reason they'd tell us as this stage. It won't help with the public appeal but it could seriously damage their case if they were to let the suspect's defence know what intel they had (and from who/where it came).
HCW hasn't brought the subject himself as far as I'm aware but it has been implied that the Investigators suspect CB went into 5A himself. Christian Hoppe of the BKA suggested CB may have gone in there with the initial intention of burglary before deciding to take MM, for example.
HCW has been pressed on 5A by interviewers and each time he clams up and refuses to give any details. He was asked whether they had anything placing CB in the apartment and responded something like "I have an answer" but doesn't want to say anything as it would lead to a thousand more questions. Mark S. asked him about the dog alerts and whether a death in the apartment at CB's hands could fit in with their theory. Again, he said he cannot comment on it. Most recently, SF outright asked him whether they thought CB was only the murderer and not the one who actually went into 5A and took MM. He responded "to the details, I don't want to tell you anything at the moment". Nor should he. What purpose would it serve at this stage, other than satiating nosey parkers and giving CB's defence a heads up on what they know?