But I think all of them were elected to the leadership of their respective organizations by their rank and file, no?
I don't think so. The press conference was organized by the FOP (Fraternal Order of Police), which is a rank-and-file organization, but the people standing up on the stage were the chiefs of the 100 or so municipal police departments in Cook County. Apparently the departments other than Chicago are broken down into three regional groups, and the three men who spoke are the heads of the three regional chiefs' organizations. Police chiefs aren't elected by their rank-and-file officers, they are typically appointed by their municipality's chief elected officer. As the heads of the three regional chiefs associations, I assume they were elected by their fellow chiefs, not by the rank-and-file officers.
They were there not as FOP members (although they may be members of the FOP) but to demonstrate solidarity with the FOP. The FOP had taken a vote of no confidence, and each of the three regional chiefs associations also too votes of no confidence. The dog and pony show was to show the media that the FOP and the chiefs associations are in agreement on the issue.