The definition of racism that you cite (“prejudice, discrimination, or antagonism directed against someone of a different race based on the belief that one’s own race is superior”) is so narrow that many of the things that get called racism nowadays wouldn’t pass muster.
Well, that’s the dictionary definition. Let;s not change it.
There is a difference between racISM and discussing things that are racIAL. The custom today is to conflate the two, and just call everything racISM.
Your example regarding marijuana is a good example. It’s being called racISM because strong illegality laws regarding marijuana have a racIAL effect. But, that effect is second order; it is not the fault of the person who supports strong marijuana illegality laws that the enforcement of those laws seems to snap up, on a per capita basis, more af-americans than white americans. They simply support a law, which prima facie, should not affect any one race over another.
The reasons why it does affect one race over another are an entirely different matter, worth studying. But they are, in fact, different matters.