They can’t. An ETF is a claim on Bitcoin, not a new Bitcoin.
When someone buys a spot Bitcoin ETF share, the fund has to buy a real coin and lock it in custody. That moves an existing sticker from one pocket to another. It does not add a sticker to the box. The 21 million cap still lives in the software, and no fund, bank, or government can change it by launching a product.
Paper promises can multiply. Coins cannot. If an ETF ever sold shares without holding the coins, that would be lying about the stash, not printing Bitcoin.