The article doesn't say: The six-hour-long interview was conducted by Bureau of Criminal Investigation (BCI) special agents and a Greene County Sheriff’s detective nine days after the January 2017 alleged murders of William “Skip” Brown and Sherri Mendenhall in Yellow Springs. Merrick’s defense attorneys Dennis Lieberman and Michael Pentecost filed the motion to suppress evidence, claiming Merrick’s rights had been violated when law enforcement officials failed to give him a Miranda warning until an hour and 45 minutes into an interview.
Miranda warnings are only required if an individual is subject to "custodial interrogation" and the government seeks to introduce his statements.
In determining whether an individual is in custody, “[t]he crucial question is whether, considering all the circumstances, a reasonable person in the defendant's position would have believed that he was in custody.... Thus, if the defendant reasonably believed that he was not free to leave, the interrogation occurred while the defendant was in custody, and Miranda warnings were required.” In making that determination, we consider various factors: “(1) the place of the interrogation; (2) whether the officers have conveyed to the person being questioned any belief or opinion that that person is a suspect; (3) the nature of the interrogation, including whether the interview was aggressive or, instead, informal and influenced in its contours by the person being interviewed; and (4) whether, at the time the incriminating statement was made, the person was free to end the interview by leaving the locus of the interrogation or by asking the interrogator to leave, as evidenced by whether the interview ended with an arrest.”
Commonwealth v. Hilton, 443 Mass. 597, 609 (2005) (recounting and applying federal test).
A voluntary interview can turn into custodial interrogation without there being a formal arrest, but the article doesn't give much info but I'd bet he wasn't under arrest at the start. Otherwise, the state's argument is incoherent.