That's a rather recent redefinition of "racism". Recent enough that most general dictionaries do not reflect it.
And one does have to wonder about the purpose of that redefinition, given that the old definition was more consistent, while still allowing to express nuances in the usual way, by adding qualifiers (so you could distinguish between "racism" and "systemic racism", for example).
So the only thing that the redefinitions seem to achieve is reducing the number of people to whom the social stigma that is (rightly) associated with racist behavior applies - by saying that something is not really racism, you implicitly claim that the stigma is not really applicable in this case.
When using this framework, one can be prejudiced against white people. But racism is about racial prejudice in the context of broad cultural and power structure imbalances.
By this definition, discussing caricatures of white subarbanites is indeed prejudice, but it's unlikely to meet the bar of racism.
This is a similar argument to why erasing "Black Lives Matter" and replacing it with "All Lives Matter" is seen as offensive, because it avoids acknowledgement of the relevant present and historical power imbalances.