I really really think its true.
She's looking at fanfic, but it could just as easily be japanese anime. Or gaming communities.
There's a really closed, narrow set of interests going on. And going on to an obsessive level. We even refer to many of these interests having a 'cult following'. For good reason. It is to the exclusion of all else. It is all encompasing and emersive. Whole new worlds of possibility. Sci-fi and Fantasy Fiction. But the key point is the OBSESSIVE element.
The common theme is they involve living in a fantasy world which offers promise and opportunity and the escape from real world and reality. It offers an opportunity to disengage from the real world and to do it with other people. You can be someone you aren't and can reinvent yourself in multiple ways.
Its not something that people who are well adjusted and happy with their real world life do to this degree. Loneliness and not being understood by others are a theme. If you are engaging in person with others DOING something, you can't live in a fantasy type world that doesn't exist. Note the group MOST affected are not the Sporty Social Group at schools who have activities to do. Nor do they tend to be in the Popular Kids Social Group. Nor kids that have to go out to work from an early age.
Look at the parallel trends in our society, which I think give you something of an incite into the why of some it. Its correlation rather than causation, but I do think you would be a fool to ignore them.
We have the known narcissistic trend on social media of the rise of Main Character Syndrome. At a time where children are indulged and parents struggle to say no and give children boundaries.
We have a rise in Incels who are basically disaffected youth.
We also have a rise in apparent issues with neurodiversity. Is this is real rise? Or is it a consequence of lack of real world stimulation? I find it FASCINATING that one of the best coping strategies for ADHD is having a walk or run - its one I know a few people with ADHD use and DS's school use it on a occasion. And its a time where kids do less exercise and walk less than ever before. Again in parallel we are seeing more and more addictive gamification of life generally where you get dopamine hits from TV, Internet and Gaming. Even the way social media works is gamified - the refresh function where there's constantly something new. There's a question as to whether this is triggering something in the way our brains develop.
We have increased social isolation and demographic destruction of communities trends too. Young people have even less opportunies to meet in real life. Restrictions on budget, mean more can't afford to go out and meet in clubs and bars in the same way they did. And younger kids aren't able to go out due in the same way to over protective parents or a simple lack of places to go. Gentrification and smaller family units also play a role in the lose of family and established community participation and importance.
We have high youth economic inactivity or unemployment. Its known this causes political destablisation and carries the risk of social unrest in society when this hits about 25%. (Look at the numbers for 18 - 25 year olds not in education).
We have a trend where children are less likely to maintain the standard of living of their parents due to economic pressures and shrinking opportunites (numerous causes for this). Societies which have been facing this for some time, have higher mental health problems generally which may be down to greater socio and economic pressures placed upon them.
THEN look at the reaction to cutting off the internet or restricting access to under 16s. Is it proportionate or rational? Or is it more like the reaction of an addict?
What is always the first advice people on here give to parents who have kids expressing interest in being trans? To check their social media usage.
I don't think the issue is purely about social contagion.