Like discussed on the Engadget version of this same story earlier, this is basically like saying that because Gab runs on the Mastodon codebase that Mastodon has a fascism problem. Good server administrators already block 90% of the problem servers and quickly block the rest through user flags. The problem servers are apparently are mostly in Japan. (Sounds like Japanese authorities need to crack down better.)
«To conduct its research, the Internet Observatory scanned the 25 most popular Mastodon instances for CSAM.» Are you saying that the 25 most popular instances aren't good? Or that what the researchers found is the last 10%, after 90% has been blocked? Or something else, and I misunderstand?
The problem is the IWF refusing to allow developers to use existing technologies to block child abuse content.