No, you don't have to use CoovaAP. Indeed, you can use any access point if you are able to bridge the WiFi network into a Ethernet port. The key is that chilli is controlling the visitor/guest network at a layer 2 level (meaning, it does DHCP for the clients). It is sometimes easier to run CoovaAP or a firmware that already has chilli support (like open-mesh, etc). See the Blog for some other examples too.