> If people (who don't know anything the bankers don't know) value the stock at $24, shouldn't there also be competing bankers willing to offer $20 a share for the whole lot, igniting a bidding war that converges roughly near the real price?

Yes, though with a discount factor applied due to the risk involved. And particularly for a high-profile IPO like $SNAP, and one in which a lot of the sellers on Day 1 will be casual retail investors placing market orders[0], it's hard to gauge exactly how much hype there will be.

[0] ie, as opposed to limit orders