The source code leaked is all custom code that hasn’t been updated since 2015 and uses functions that have been removed from PHP for being insecure since 2019. The hack supposedly took advantage of PDF uploads not being scanned for embeded code. 4chan uses a program called ghostscript to create thumbnails of uploaded PDFs but the version they use is from 2012 and the hackers likely used a known exploit to get it to run embeded PDF code.

So unless the other websites are also running software from a decade ago, they’re probably good.