Are you sure on that? (I know nothing other than these results)If you sell covered calls that are in the money when you sell them, it restarts the long term gains clock on the underlying shares. If you actually have long-term stock this is a disaster. (In the US, taxable account, IANATL, not advice.) You can sell OTM calls though.
What Are The Tax Implications of Covered Calls? - Fidelity
A qualified covered call is a covered call with more than 30 days to expiration at the time it is written and a strike price that is not "deep in the money." The definition of "deep in the money" varies by the stock price and by the time to expiration of the sold call.
According to Taxes and Investing (page 23), "Writing an at-the-money or an out-of-the-money qualified covered call allows the holding period of the underlying stock to continue. However, an in-the-money qualified covered call suspends the holding period of the stock during the time of the option’s existence.
You might be thinking of what happens if they are exercised:
How Does a Covered Call Strategy Increase Your Taxes?
You may hold stock that you have owned for more than one year, have significant gains in the stock and you want to sell calls against the shares. If you sold the shares outright, the gains would be taxed at the lower, long-term capital gains rate. If you sell deep in-the-money calls against your shares, you negate the long-term holding period for capital gains. Deep in-the-money calls most likely will be exercised, calling your stock away and leaving you with a big tax bill. To avoid this situation, only sell out-of-the-money calls against your longer-term stock holding.
Other technicalities related to dividends.
The tax impact of selling calls - InvestmentNews