I think it comes down to the quote in the OP "So while it may sound weird, it turns out that obtaining a statement outside Miranda but not admitting it in court

I think it comes down to the quote in the OP "So while it may sound weird, it turns out that obtaining a statement outside Miranda but not admitting it in court is lawful." Your assuming the motivation is to violate the suspect's rights. It might not be -- there might be a clear and reasonable aim to find out if there are others involved. The authorities probably has a lot of evidence & a strong case without the interrogation. That they want to question the suspect about other parties or networks seems reasonable. It's a tactic, but perhaps their primary motivation isn't to interrogate to get a conviction -- it's to garner a wider net of information. In that case, the Miranda & having the suspect clam up would be a hinderance.