I'm not a security expert, but my tech career has involved a lot of automated testing in weird scenarios, including iframe-based Facebook games and browser-based mobile apps. Automated tests face a lot of the same challenges that a malicious third-party would, so I know a little bit about how to get past them -- or rather, how to deliberately create vulnerabilities (in the dev build of your system) so that your tests can get past them.
Edit: I am curious why someone downvoted me on that one though. I can understand how my comment about the ban being dumb but TikTok also shipping a keylogger could anger people on one side or the other. But just explaining how in-app browsers revive a security problem that's been long-solved in standalone browsers?