Two questions:

Even after "reviewing millions of videos on the biggest international porn sites" how can they get the 90% figure?

Only looking at the biggest sites is going to get you a biased sample. Not all porn is on big sites. That's not to say there isn't a problem.

And did they talk to sex workers/performers?

Governments and anti-sex orgs have a habit of talking about and legislating sex work without actually listening to those who do it. In March, the UK’s All-Party Parliamentary Group on Commercial Sexual Exploitation published a report[1] labeling all porn exploitation without speaking to any sex workers.

[1] https://www.vice.com/en/article/y3pa9y/porn-sex-workers-expl...