No. He had already started to contemplate Satyagraha in the early 1900s in South Africa. When the British declared war on the Zulu in 1906, Gandhi was part of an Indian stretcher-bearer corps to assist in the war effort during the Bambatha rebellion.

During this time he noted that militarily it would be impossible to fight against the sheer firepower of the British army, so he had steadfastly adhered to non-violence thereafter. Coming back to India, by then he had already refined his concept of civil disobedience and Satyagraha, so he went along with that since it was compatible with his inherent belief of Ahimsa.

His recruitment flier in June 1918 is regarded by some as something of an inconsistency with regards to his stance on non-violence, but Gandhi believed that a person may resort to violence to defend themselves if they lack the strength to non-violently resist (Which basically involved enduring corporal punishment, incarceration and so on.)

Bear in mind that Gandhi was still a loyalist at the time, so he wrote that flier believing that Indian participation would pave the way for establishing Home rule in India. After World War 1 however, the sentiment changed, and he adopted more of a nationalistic stance to push for independence. His non-violent beliefs were a pretty big pull for the common man, and it called forth public and nationalists alike to unite together.

As for the other nationalists, not all of them believed in non-violence either. In fact, nationalists such as Subhash Chandra Bose believed that armed conflict and violent resistance was the way to go. Sure, to some extent, not having access to arms would have limited their options, but a group of 50 well armed officers would be hard pressed to fight off a mob of 800 people with torches. Provided of course, you could unite them to fight together in the first place.