To people raised under that philosophy, it can be disconcerting to realize that now it has become proper to judge everything only after first assessing the skin color of who said it. This isnt as shocking when someone makes a statement related to race, but the point is really driven home when the issue at hand is a tree.
So maybe the word is relevant. It’s not obvious to me what the relevance is, so asking seems pretty logical. Otherwise a reader is forced to conclude that it somehow has to do with white, presumably Australian stereotypes.
Likewise, if Perth is included it clues the reader into a more specific area which might tell us something or not, depending on our pre-existing knowledge of Perth. No reader is going to conclude that the detail Perth was added merely for “flavor” and has no other significance. They might not have asked even if ignorant, because they might’ve made their own assumptions (e.g. this is an area where the trees can be found), but they wouldn’t assume it was a meaningless detail.
And yes, I think I might have asked about relevance if the poster had said, “My roommates, while wearing purple t-shirts, told me...”
When you add in the baggage that comes with race and racial stereotypes are you really surprised anyone asked? You don’t even have to be offended to not understand why race was included.