What kind of colonist are you referring to? I don't think he's talking about the same thing you are, but I don't want to speak for him or put words in his text.
I took it to mean that the term white (or the classification) is a made up construct, and to refer to oneself as that (or whatever) is holding yourself to a weird standard that those in power created to classify us/divide us, etc.
Decolonize, to me, just meant to not believe or accept the standard that other people created for us as a people. I.e., Someone using a definition that was created to artificially separate us as people.
Someone saying that "white people" do this or what, doesn't offend me. In a lot of cases, it's true, but even if I find it untrue (or it's not true about myself), it doesn't mean the other person is wrong.
I think a better way to say it is (and I admit this might totally not be what dviss1 is getting at, so forgive me if it is wrong).
Like if you know a Lakers fan who is knowledgeable, affable and not an arrogant clown, but you say "man, "Laker fans" are idiots", he or she would probably go "you're right. Many Laker fans ruin it for those of us who can name players outside of Kobe, Shaq, LeBron and Anthony Davis".
So if you're *not* an annoying Laker fan, someone referring to a subgroup as a Laker fan, isn't about you.
So if someone says "that's such a white man thing to do" to someone else, it doesn't mean I'm that. Mostly because I don't consider myself 'white' because it's a made up classification.