Sorry for the delay, the install video and description on the page make no mention of the other RCA, but I can clearly see it in the video. It looks like it has a label on it too. It "could" be the wiring for the cargo camera pins, but without seeing the label or the pins it connects to I can't be sure.
A quick test for you (if it's not labeled) would be to enable the cargo camera with AlfaOBD and then plug the RCA from your infotainment kit for the backup camera into that blue connector to see if it shows on the cargo camera input. This should work because unlike the factory camera this kit shows it's powering the camera with a keyed source (cigarette port/charger plug)