It seems that almost everything in the PCA document is information they had back in 2017.
It was presumably too weak to charge him then. And presumably it was too weak to obtain a search warrant until recently. Something changed during the October interview.
I would guess they had the bullet from the crime scene, it was not the murder weapon, or even fired, and they had no reasonable cause to search RA's property for a weapon matching the bullet.
Then on October 13th they re-interviewed him (and/or his wife) and got him to admit he owned a gun, possibly even one of the same bullet type. That might have tipped the scales enough to obtain the search warrant. To obtain a search warrant you have to specify what you are looking for, where you will look, and explain why it is reasonable to expect to find evidence there that is linked to the crime.
Then the search turned up the gun. Then they matched the gun to the bullet forensically. That would be the first evidence they had that circumstantially placed him at the scene of the crime, not just on the bridge or walking away.
That would seem to explain why they could not arrest him in 2017 and why the PCA is just barely different now, and just barely enough to arrest him now.
I disagree with those who presume they had a bunch more evidence at the time they arrested him. And the statements from his defense attorney suggest they have not yet revealed more evidence such as murder weapon, DNA, clothing, blood, etc.
However, it remains possible that further searches after his arrest could uncover more direct evidence. They will want more evidence to assure a conviction.