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This might sound ridiculous but it's taken very seriously by a growing number of people; it has its roots in a certain strain of postmodern thought called critical race theory that's been slowly emanating out of academia for the last decade or two and is now seriously spilling over into the mainstream. See e.g. Robin Diangelo's book White Fragility (which is currently back on the bestseller lists) which asserts that all white people are racist, and if you're white and don't admit that you're racist then this is just you demonstrating "white fragility" which proves... that you're a racist. This is of course divisive, harmful, unfalsifiable garbage but if you don't think these kind of ideas are becoming very influential then you haven't been paying attention.
So basically the reason why it seems like people are constantly throwing around terms like "racist" and "white supremacy" like they have no meaning is because they have consciously and deliberately stripped these words of all meaning. Of course that's not how they'd describe it, but if think this is just me being a conspiratorial right-wing nutjob I highly encourage you to learn more about critical race theory (newdiscourses.com is a good place to start) and understand what these activists are saying. This stuff is coming for you eventually whether you like it or not so you'd better be prepared.