Only one was an ex-cop.
In order to make a citizen's arrest, you need to witness the crime. Neither of those two vigilantes did and just went off of what was reported to them by the neighbor.
There is multiple security footage for Arbery going into the property for the past few days/nights, (which is what I'm assuming the defence will state) but he doesn't take anything and entering undeveloped property isn't a crime I think.
The real crime is why this wasn't pursued earlier. Apparently the ex-cop was well known in the department, and they swept it under the rug until it became viral.