Yes in Best Actress, you can win with a weaker film, but Julianne Moore was in a very dead year. She probably wouldn't have won in a more competitive year. Meryl is Meryl so rules don't apply for her. And The Danish Girl was a BP nominee, so at least the academy liked it (plus not like Steve Jobs was a particularly strong film either).
In more recent years, when there's been a more competitive race the stronger film has won out. Look at Olivia Colman winning over Glenn (😭😭😭) and Fran picking up her third over Davis and Mulligan. Though, I guess those were also stronger films with the Academy and Parallel Mothers wasn't as much.