The key sentence is > First discovered by Kaspersky researchers in early 2022, some of the backdoor samples were still not flagged as malicious in most popular

The key sentence is > First discovered by Kaspersky researchers in early 2022, some of the backdoor samples were still not flagged as malicious in most popular online file scanning services. Of course by now majority of AVs detect those files, detections spread rather quickly between AVs. Sadly, VirusTotal seems to show detection history only to registered users. I can confirm that at least second sample still had 0 detections in April.