The issue comes down to figuring out when exactly Harris was taken into custody because there's a difference between what you voluntarily tell police and what they can ask after they've read you your rights. Police talked to Harris October 1, after they matched his hand-print to the bloody one found in the Gee's master bathroom. That's the interview prosecutors want the jury to hear since Harris says the print isn't his.
We now know that was a lie which helps build the state's case. But, anything Harris said before he was technically under arrest can't be used in court against him. That's where there's a gray area.
At the beginning, police told him he was not in custody. By the end, he was. Both sides agreed the jury will get to see that interview.
This isn't the first hiccup in the trial. Because of editing issues, Harris' first interview with police also had to be re-cut to take out parts the jury couldn't hear.