In fairness....she's not a girl anymore but that's a relatively recent fact. She's 19, I think.

It would not have been inappropriate to call her a girl before she turned 18 (or whatever the legal age of majority is in Sweden), she was one.

Let's not also forget that she effectively uses her status as a child/teenager as part of her activism. "What are you leaving us, the youth of the world? We'll never forgive you if you leave us a dying planet!" was both her rallying cry for change and her criticism of world leadership. That would not have been effective if she was seen as anything other than a child/teenager.

More important is that they were using the term as a gendered agist slur; promoting the notion that her activism and ideas should be automatically discounted, ignored, and even rebuked BECAUSE she's a girl/young woman.