I agree with your train of thought, but I disagree with the specifics.
Though, that's in part due to my own definition of discrimination in this context to be "placing a label to tell 2+ groups apart and taking action using that label".
Taking money from the rich and giving to the poor is inherently discriminating between the rich and the poor with a follow-up action to remove resources from one group (the rich) and give to the other (the poor). The action is there regardless of the reason or ethics of the context (i.e. why someone is rich or poor).
If someone really doesn't want to discriminate, sure. But I think the argument of "labelling it discrimination is wrong" is wrong in the sense of my own definition of discrimination. For me, a more useful argument would be "well, let's figure out the metrics we're optimizing when we discriminate like this".