tl;dr Castro fits the definition of a dictator, but leftists are probably challenging the assumptions attached to that title.

I think there's very little argument to be made that Castro did not fit the common definition of 'dictator.' He assumed power with the help of his revolutionary military force, and ruled an one-party state until his death. The level of power he wielded singularly, meets the criteria we would conventionally ascribe to a dictatorship.

The leftists to which you're referring are probably reacting to the connotation associated with the title of 'dictator' rather than the basic facts of Castro's rule. That is, they are contesting the idea that his rule by dictatorship means we must necessarily view Castro's reign negatively. They would probably point to his relative popularity, particularly among the rural poor of Cuba, as well as the benefits like education and healthcare extended to citizens. They may also argue that transitioning to democracy during Castro's rule would have endangered the socialist project by inviting foreign interference and destabilizing the revolutionary government.

These points arise from leftist critique of conventional understandings of what makes a government 'good', and challenge us to think about how we define freedom. Interrogating the assumptions attached to words like "dictator", "democracy", etc. is always healthy. But as a matter of historical inquiry, I am relatively confident that Castro meets the definition of a dictator. Other historians feel free to chime in if I'm wrong.