The president can't unilaterally decide whether tips are taxed or not either.
This article is not discussing an action Harris can or will take. This is discussing policies she supports- she can use political influence to help get it through and past Congress.
This applies to like 95% of all political discourse by the way. People want to blame the president because it's easier to blame one name and face than to try to pick out one of the hundreds in Congress.
Maybe this is just me, but in the past two decades in which I've been politically active, and in what I've studied of the past, I have never seen anyone ever support a strategy of trying to move already-elected politicians further left.
If anything, the discourse is usually about how presidents look more conservative than they are because conservatives have had control of Congress for all but a few months of the past 40 years.
Biden ended up being further left than I thought he would be, but I don't remember anyone suggesting pre-election that he could be pushed further left.