When I say, it is not there in Mahabharata, I mean that I was not able to find the exact words in Mahabharata.
But the spirit of the words 'dharma himsaa tathaiva cha' can be defended to be not in contradiction with the spirit of the book Mahabharata which in turn is not in contradiction with the Vedic worldview that is in the upabrimhita form in the Panchama Veda.
Himsaa has two meanings : 1. Any violence in general 2. violence that is not justiciable (violence inflicted on the helpless innocent persons)
Violence (in the meaning of any violence in general) committed in order to punish or prevent a person resorting to violence that is not justiciable (violence inflicted on the helpless innocent persons) is considered to be nonviolence 'ahimsaa' (nonviolence) only.
Kshatriya is viewed as the one who saves people from getting kshati. A kshatriya's violence is violence (in the meaning of any violence in general) committed in order to punish or prevent a person resorting to violence that is not justiciable (violence inflicted on the helpless innocent persons) and as such is considered to be nonviolence 'ahimsaa' (nonviolence) only.
Kshatriya's violence is 'paritraaNaaya saadhoonaam vinaas'aaya cha dushkritaam'
Ahimsaa in the meaning of no violence of any kind
and himsaa in the sense of violence (in the meaning of any violence in general) committed in order to punish or prevent a person resorting to violence that is not justiciable (violence inflicted on the helpless innocent persons) which in this sense is considered to be nonviolence 'ahimsaa' (nonviolence) only
are distributed among varNas and aas'ramas.
There are mantras in the Vedas ending in 'sugrichcha' and also those ending in 'maa himseeh'. This apparent contradiction gets resolved when the Vedic worldview is understood from the above described perspective.
But defending that the exact words are available in Mahabharata is what I leave to someone from Swami Chinmayananda's organization