What they mean is THEY will not support it, meaning they will not field phone calls or chats from you to try to get them working.
Same thing with trying to use a different brand camera - it will probably work if it is an ONVIF compliant camera, but they don't want to field the support calls for it, so they say only use Lorex cameras.
You will find in many instances ONVIF doesn't mean much.
Even as a standard, there is no "standard" that every manufacturer follows.
Audio rarely follows a standard. Things like AI triggers, heat mapping, people counting, etc. rarely follow a standard.
In reality, ONVIF is a paid designation, so a camera can set up their protocol however they want, but if they belong to ONVIF, they can slap ONVIF on their stuff. Some manufacturers are better than others at providing a standard that can be used with other devices.