If I sold short 10 XYZ, then the stock split 10:1, then I bought 100 XYZ to close the trade: On the IRS 1040 schedule D, am I correct to report that I traded 100 shares of XYZ?
Yes. Report the share number on the date you closed out your position. That's when you "realized" your gain/loss.
A Stock Split is the increase in number of outstanding shares of a company without any change in the shareholder's equity or market value. There are no tax consequences when a stock split occurs, but you should be aware of pre-split and post-split prices for tax calculations of determining gains/losses.