I think it's quite possible Emma Thompson didn't 'know' as in having enough actual factual knowledge to say anything. I encountered at least two, I'm fairly certain three predators in my late teens (in completely different contexts, I am not a Hollywood star! But nevertheless contexts in which the men had considerable power over younger people). I was not confident with men and assumed that the lack of attention they paid me was that I was not attractive enough. I knew they were flirtatious with other girls, I heard the odd 'off' comment but had I voiced concerns it would have amounted to nothing concrete, more a vibe of 'dodginess'. Where I could though, I advised others to be careful.
Looking back I realise these men were abusive and twenty years later have heard concrete stories from people I knew at the time but I genuinely did not know enough then, even though if you looked at the things I was doing, people I was with, you may well feel justified in saying I must have known, should have spoken out.
The reason is I think that these men are clever in their targets. I realise that these men had in common that they all knew that when I wanted to I would speak my mind and they knew that in each situation there was someone else who would believe me and also not be afraid to stand up. In other words they were not confident enough that they could manipulate me to say nothing so made sure there was nothing I could say.
Several years later, one of the men was sacked for his behaviour. I met someone at a party who told me about it and clearly thought I either would have 'known' something or been a victim. I remember telling him I personally had nothing concrete to go on but it didn't surprise me one bit.