Is it possible that hedge funds like Blackrock can load bitcoin immediately upon ETF approval?

Is it possible that hedge funds like Blackrock can load bitcoin immediately upon ETF approval? Is there a rule where they can't buy until their customers buy into their funds? I imagine if a bunch of ETFs get approved at once that they aren't going to wait around for other funds to buy in unless there's some rule preventing them from buying bitcoin. If the ETFs get approved it's almost like a stamp of approval on the asset and should remove any hesitancy from investment firms to start loading up. Am I stupid or what am I missing?

I'm trying too look at the other possibilities from what others are saying about BTC ETF approval is a sell the news event because funds won't come in till months later.