Roy had fallen into the Void once, so he knew very well that time still existed in the Void. There might not even be space in the Void, but there definitely was time.
Therefore, he was certain that he could Voidize in this dimension of time.
Once Roy became a Void creature, the power emanating from him would corrode all the matter around him. This was due to the characteristics of Void power. Although Murozond had the home ground advantage in this dimension of time, he was still a part of the material world. Any attack he made against Roy would cause him to suffer corrosion from the Void power.
Roy might not be able to affect the dimension of time directly, but he could indirectly affect it by changing Murozond. As long as Murozond was corroded by the Void and couldn't stably use his time power, this dimension of time might be unable to continue existing.