It is most unusual for a Chinese ambassador to write an official letter of complaint to a foreign newspaper. It seems particularly inept to do so on a subject that Beijing would rather see buried than highlighted: namely mass human rights violations.
It would appear that the government in Beijing protested to the authorities in Moscow at Russian press coverage but that the complaint was deflected with the suggestion that the Chinese instead approach the press directly. The Chinese ambassador to Russia, Zhang Hanhui, has a reputation for being high on morality (or, on another view, hypocrisy): just less than a year ago he complained that the Russians were pushing back over the border Chinese citizens who had crossed into Russia. The Russians naturally did not want the Wuhan virus coming in through uncontrolled immigration. At that time, last April, Zhang suggested that the Russians were contaminating China by sending their citizens back and that this was immoral! He has now written a lengthy and pompous letter to Nezavisimaya gazeta complaining at its coverage of Uighur persecution based on publications in the Western media. If he is to be believed, it seems the Russians (and everyone else) have it all wrong and that in fact all the Chinese government is doing is taking measures necessary to suppress domestic terrorism. So that’s alright then.