That's right. Instead of stupidly insisting it wasn't possible, even though he knew the police had found it there, he could've come up with some explanation that made it POSSIBLE if unlikely. They had just told him about them finding it and even SHOWED him HIS bullet, so he knew he had no chance of convincing them that didn't happen, but he still kept claiming it wasn't possible. When he should have been trying to think of some way that it might actually be possible, some innocent way. Because if he doesn't give them some alternative, they're left thinking exactly what they told him they did think. But all he gave them was an impossible explanation. So they had two explanations to consider: one was impossible that RA gave them and the other one they came up with themselves after finding the evidence. If he was smart, he'd have seen that they had no other explanation to go with, and the one they had, made RH guilty.
But we know he was not smart and didn't furnish them with any alternative. But he didn't even try! He just kept repeating it's not possible, which was only possibly true if the cops were lying. But the cops knew they weren't lying, duh.