OverlappingRanges feature is enabled by default, this way:
https://github.com/k8snetworkplumbingwg/whereabouts/blob/master/pkg/config/config.go#L38

Later on, the config file (flatfile) is processed and those values are merged to the actual IPAM config struct.
Once this OverlappingRanges is set to True, we cannot override it to False. I believe the problem is that a boolean variable has no "undefined" value, just True or False. In case of False, we don't know whether it is unset or intentionally set to False.

So we cannot disable the OverlappingRanges via config file, we need to disable it in every NAD, which is not a feasible option.