I don’t know about Pitt… this was well after many big movies like Legends of the Fall, Seven, Interview with a Vampire… he was a pretty established movie star by then and worked with many big directors / stars so had solid connections in the industry.
Godfather Spielberg may have given Weinstein a call if he didn’t want to cast her directly as too obvious nepotism before the nepo baby era.
After he cast her, Weinstein may have just wanted to get the movie Oscar exposure, why wouldn’t he? Everyone does that for their movies. And he didn’t have to work hard since she was becoming an icon as half of the hottest couple back then, the short hair, the Tom Ford for
Gucci suits…
Not saying it’s impossible she slept with HW but there are also other plausible explanations.