You know how there is a scientific definition of theory that is different than the common usage of theory, and it is really confusing when two people are using the different definitions interchangeably?

Same thing with racism. More than one definition which causes confusion when used interchangeably.

There's more to racism than a dictionary definition, you know that right? People didn't dedicate their lives to study the topic just for the fun of it. The realities of racism from the dominant group (white people in the west) and racism in the marginalized groups are very different. Racism is bad no matter where you fit in that spectrum but the harm done to certain (non-white) groups of people is more severe by several orders of magnitude

What word would you use for the systemic oppression of racial minorities in America?

Do you feel white people experience systemic oppression? In what way?

Which racial group has historically been in control of the large structures in our society (government, large and wealthy corporations)?

If you’re wondering why I’m asking these questions, it’s because I think you’ve missed the point that the poster you’re responding to is making. You seem to be arguing about which words to use instead of noticing the wider topic.

Dictionaries are often treated as the final arbiter in arguments over a word's meaning, but they are not always well suited for settling disputes. The lexicographer's role is to explain how words are (or have been) actually used, not how some may feel that they should be used, and they say nothing about the intrinsic nature of the thing named or described by a word, much less the significance it may have for individuals. When discussing concepts like racism, therefore, it is prudent to recognize that quoting from a dictionary is unlikely to either mollify or persuade the person with whom one is arguing.

https://www.merriam-webster.com/dictionary/racism