Just my take on this. It's likely that to access knox you need to have a valid security key for the API. AdAway is open source. Samsung would frown on the api being shown in the source. AdAway would be forced to close part of their source. This would probably mean it could no longer be available in fdroid. This would make users sad.I have a feature request.
With DOH support coming and hosts file for rooted and local VPN for unrooted already supported there is yet another way, using Samsung Knox for unrooted Samsung devices.
Disconnect Pro in the Galaxy Store does this method.
This is better than local VPN for unrooted Samsung devices.
This is only my opinion and might be very wrong.