I agree.
But my presumption, which I believe is not contradicted by the PCA or anything shared by LE, is that LE had almost all the information in the PCA in 2017 within days of the crime, and they did not have probable cause for a search or arrest or they would have done so. (I do not believe they missed or forgot the information provided by RA and witnesses that were provided in the first days following the crime.) Obviously they had the bullet, and the recording in which one of the girls said "gun", plus RA's admission he was on the bridge and most likely a very good idea that he might have been BG.
So last month, following the second interview, it's reasonable to think he said something revealing that could provide probable cause to search his property or his cell phone records, the probable cause they did not have for 5 years. That may have been as simple as whether he even owned a gun, especially if he finally told them he owned the Sig with the same bullets as the one found at the crime scene. That would add enough to finally have probable cause for the search. And upon finding his gun and matching it to the bullet at the crime scene, they would finally have probable cause for an arrest, consistent with what I gleaned from reading the arrest warrant.
(I do not believe LE withholds evidence from an arrest warrant trying to have just enough for an arrest. They explain all the evidence they have that could contribute to probable cause for the arrest. Anything they did not include in the PCA was probably just too weak to bother with, not super strong evidence they want to hide.)
Some of the above could be wrong, but if it is correct then the bullet is THE key piece of evidence that justifies the probable cause for the arrest which they presumably did not have for five years. It seems to be the only thing that puts him at the scene of the crime.
Otherwise all they had was suggestive evidence that he was on the bridge. Walking on the bridge is not a crime. Seeming creepy or intimidating is not a crime. Saying "Down the hill" is not a crime unless there was also a threat or provable attempt to force them to do so. The girls saying "gun" makes it seem like they were saying he had a gun, but it's not a slam dunk like if they said "do what he says he's pointing a gun at us" - just for argument sake they could have seen a bulge in his jacket or just worried he had a gun because of his tone. If there was clear evidence of a crime on the bridge (kidnapping) and probable cause that RA was BG, they could have easily gotten a search warrant that we know they did not get (or failed to get) for five more years.
So my question is, just how big of a coincidence is it that a bullet was found at the crime scene? Are the woods in Indiana full of bullets? When I go hiking I hear gun shots all the time, and many people I meet have guns and ammunition. I don't assume anything about whether a bullet near the crime scene is just a coincidence because they are everywhere, or highly unusual like finding murder victims thus clearly something the murderer left behind.
The rest of the case against RA is compelling but circumstantial, not so strong that LE stopped investigating others. They switched to focusing on RL and later KAK as prime suspects. You could argue of course that they were just being thorough by investigating everyone, but the RL warrant says LE believes RL was the murderer, not that he was the second best suspect.
The bullet, I contend, is the difference. So it is critical to know how unusual it is to find a bullet anywhere in that part of the country, how unusual it is to match RA's gun, how strong the forensics are that match the bullet to the specific gun. It would be fairly easy to scour a square mile of the park away from the crime scene and determine that no other bullets were scattered randomly on the ground. Or if I were on the defense I might go out and see if I could find hundreds of them no where near crime scene. With so much riding on that bullet (imo), I'd want to know everything about it.
For those who believe the bullet was just another contributing factor to the probable cause, it would be interesting to know whether a search warrant or an arrest warrant for RA was attempted and rejected by a judge prior to the bullet/gun connection to RA. I suspect that failed attempts at warrants are never public, but maybe they are public records or fair discovery once a successful warrant clears? Does a lawyer on this forum know the answer?