I suspect your correct, it has been on my to do list to remove channels that a camera reports that it does not support. It appears you have supplied the API call to check if the camera supports the IO features and given the response already, can you confirm it is complete as it looks cut short. That is all I need to complete this.
For example mine gives
http://admin:xxx@192.168.1.35/ISAPI/System/IO/capabilities
<?xml version="1.0" encoding="UTF-8"?>
<IOCap version="2.0" xmlns="http://www.hikvision.com/ver20/XMLSchema">
<IOInputPortNums>0</IOInputPortNums>
<IOOutputPortNums>0</IOOutputPortNums>
<isSupportStrobeLamp>false</isSupportStrobeLamp>
</IOCap>
EDIT: @Tommo it would also be smart to install the latest firmware, see the link I get them from if the camera is an older model. Mine is on firmware 5.6.6