I’ve seen police pull this one by design before. They’ll interrogate a suspect, suddenly “remember” to Mirandize them, then pull out the old “well, you already told us everything. No point in hiding it now. You might as well admit it on the record.” Shady? Absolutely. Admissible? Probably.
You could try to make a case that the whole routine sort of un-Mirandizes them, and claiming that “you’ve already admitted it, so saying it again can’t make matters worse” undermines the idea that statements can/will be used against you, but that’s a hard sell, at best. Might work in conjunction with the idea that the defendant was in a state of diminished capacity during the interrogation, though.
And of course, I immediately saw the very next page. Oops.