This is it exactly: Apply the same rules that they have today for content moderation, but allow users to opt in to that filtering. Even if they apply it by default, that’s probably enough of an fig lead to get by this ruling.
They can provide a first-party/recommended filter, but like with default browsers, search engines, or other preferences, the app should present the user with the choice in clear terms rather than silently applying the default. Defaults are powerful, so having the app apply its default first-party filter and hide away the option to change it in some menu would be approximately equivalent to changing nothing but forcing Twitter to act as a datastore/api for competing microblogging platforms.