IMO, yes, I think that's exactly what they're doing. They know by now that Casey will never admit to a lie until she is absolutely backed into a corner, and I think that's what LE is trying to do. "Look Casey, we KNOW that she is dead, and we can prove it to you. You can't keep saying the nanny took her. Now 'fess up already!!!"
Although, the whole Anthony family keeps denying that she is dead even faced with this forensic evidence. I feel like we've hit a brick wall here in the past few days ... and I think it's going to take TES (or anyone) actually finding the body or the "dump site" to make any progress at this point. And if they don't, then I fully expect the Anthonys to keep up this crazy charade until Casey is charged and, ultimately, convicted of harming her daughter.