I personally don't know what Meryl did or didn't know about Harvey's crimes. Or Joss's indiscretions.
Same way I don't know anything about you, or vice versa. I think that Meryl Streep, like any human, is a complicated person. But as a woman who is in no fear of being blacklisted in Hollywood for speaking out against a producer (a rare position, to be sure), I'd like to think she would if she knew literal rape was going on from a producer she worked with many times in the past.
Without knowing her personally, we can only take her at her word. Maybe she's lying, but why is that more likely than her telling the truth? I choose not to be cynical and instead believe she really is a good person who is using HER position of power to platform her beliefs. It's a bad look when someone she thought was helping a given cause in the past is revealed to be a true enemy of it, but you can't apply hindsight to a past action.
People want "everyone knew, it was an open secret" to be the truth, because it's a jucier story that way to say or imply that all these other people are implicitly guilty.
To turn it back on you: you're not in Hollywood (I'm assuming), how do you know it's an open secret?
Harvey Weinstein had a certain reputation that earned him public notoriety, but it certainly was not known by most people that it crossed the line into criminal acts like sexual assault. And I think it's ludicrous that everyone who ever happened to work in a movie he produced is suddenly brought into question "if they knew". Maybe some did, most almost certainly didn't. If Harvey went around acting this way to everyone and in front of everyone it WOULD be in the open. But it wasn't before. It's clear he specifically targeted his victims (which is even more vile and malicious).
All I'm saying is that we shouldn't be trying to figure out who knew or who didn't. Who could have come forward. It's all irrelevant unless those individuals too were guilty of his crimes. Knowing and not coming forward does not make them just as bad. And moreover, most almost certainly didn't know. I'm not a fan of trying to nail anybody and everybody to the post when they aren't even the perpetrator.