I don't know the context, but I'd assume that's less an abandonment of nuance and more a different definition of racism.

I understand why anti-racists use a different definition of racism from the norm, but if you're going to do that it's very important to make your definition clear, and that doesn't always happen.

I assume they mean "Racism is prejudice by someone who is on the powerful end of the racial hierarchy towards someone on the less powerful one based on race". Such a definition admits that (often quite damaging) race-based prejudice can happen from POC, but it just doesn't call it racism (most of it, however, a result of racism). Also the hierarchy of race is considered on a global level, so regardless of the local situation only white people can be "at the powerful end".

Personally my definition is "Racism is prejudice which supports the underlying power structure's attitude towards race" and POC can be racist, but words mean different things to different people. Also while white people are at the top of the global hierarchy of race so can't really experience racism I think there's plenty of situations where one non-white group has their own local racist attitudes ie majority japanese people towards the indigenous people of Japan.

I do sometimes think these discussions center too much on whiteness and the definition of race created during colonialism: this idea and experience of race is hugely important and covers pretty much everything that comes up in everyday life, but it can be limiting when trying to put things in a historical/global perspective. But like I said to ataxi above: it's flawed, but it's useful, and I've yet to find a better way of looking at things (certainly not one endorsed by POC anti-racist activists who I tend to assume have a better grasp on this stuff than I do)