Answer is here, although the article is outdated and the most recent news is that they are rewriting the browser at least in Swift.
https://awesomekling.github.io/Memory-safety-for-SerenityOS/
https://awesomekling.github.io/Memory-safety-for-SerenityOS/
Their GitHub has 0,3% Swift code. They said they start once Swift 6 is out. It has been out for months. So either they abandoned Swift or haven’t really started or they are really really slow to start using it. All three options are against the article being outdated, wouldn’t you agree?