I couldn't find any primary sources on this, but I did find a relatively good secondary source, as AP News covered this topic at around the same time the Bluesky user posted the article from ESRT about it, which leads me to believe it's real as AP is a fairly reliable source when it comes to news, and the person who wrote it writes exclusively on topics related to this. The article did seem to have somewhat of a bias in its undertone skewed towards the Ukrainian perspective, and the author has expressed opinions biased towards that perspective on other platforms as well, but they mentioned many different other sources for quotes and information.
https://apnews.com/article/russia-ukraine-war-drones-fire-ustluga-9cb9adca61c79312b470a00fa8ae80be
However, some of the statements in the ESRT article the Bluesky user originally linked I couldn't find any sort of source for and were not mentioned in the AP News article, such as the Russian Ministry of Defense claiming that the Ukrainian army lost over 1200 soldiers in those 24 hours, and that claim itself is highly unlikely as there have been many claims from the Russian Ministry of Defense over the war that report even higher numbers without any sort of concrete data.
