Interesting. Thanks for posting. This is likely a similar system to neighboring Henderson County's NEXEDGE VHF system, which is almost entirely encrypted. I won't be surprised if this is an expansion of the Henderson Co system into Polk Co. Tryon PD and FD have been using NXDN conventional channels for some time.
Knowing Polk county, I'd highly doubt they'd be going to VIPER. Everything else for them would likely make the move to NXDN trunked like Henderson if anything-But last I was down there and a few I know from there, they are having problems even keeping up with the first gen Kenwood NX units.