How do Robinhood's limit orders work for pre-IPO shares?

I am confused by Robinhood IPO access. On one side, https://robinhood.com/us/en/support/articles/preipo-trading/ states: We offer pre-IPO orders for a small selection of stocks, and won’t support pre-IPO orders for every company that lists on the market. These orders are limit orders only, which means we can’t guarantee your order will get filled. which sounds like it is just a regular limit order on the secondary market. But at the same time, those limit orders must be passed much before the ma