Twitter doesn’t default to the “For you” tab. It remembers which one you spend most time on (I think Elon tweeted this at one point). I use chrono exclusively and it doesn’t default to the algo feed when I open the app.
At the very least, Twitter has an A/B test going on iOS which switches you back to the For You tab aggressively. It's as aggressive as switching to another app like email for less than a minute and you'll see a jarring switch as the app shifts back to For You tab.