That use of "only" before "15%" is a serious underestimation in absolute terms, but wholly appropriate when those 15% are relative to the US's self-inflicted trade damage: it's still immensely better when 15% of your trade goes to a trading partner which has picked a fight with you than when 100% of your trade goes to trade partners you have picked fights with.
Further, in the last 2 decades or so China has both been growing its internal market (so that its Economy is less dependent on exports) and diversifying the countries they export to, likely in preparation for the death throes of empire as the US falls from its top position, which were entirely expected for decades, as was that they would include some form of trade pressure on its closest adversary.
Absolutely, this is going to hurt China in the short-term (and possibly mid-term, depending on how easy or not is to redirect those exports to other destinations), but the Trump trade war with the World is going to totally fuck up the US and it's the kind of harm that gets worse the longer these measures remain in place because it will be further eating away at the US brand (so even after the tariffs are removed many customers won't be coming back) and accelerate the demise of the Reserve Currency status of the USD (which will in turn be reflected in massive inflation as those dollars flood back in and way bigger deficit related problems because the costs of debt servicing will go up when there is little demand for US Dollars abroad).
