They key point that's missing here is that ALL stock trades are on "margin" because in reality it takes 2 days to settle a trade, but that is abstracted away from you by the brokerage and clearing house, and made available to you instantly.
Caveat: I'm just starting to learn about all this, so it's probably not a perfectly accurate analogy.
Well not really, a lot of people were transferring money into Robinhood but they let you trade before the money is available. I imagine that had a huge impact on their cash crunch. Still weird they weren't straight forward with the reason or why they wouldn't enable trades for cash accounts.
Robinhood has defaulted new users to "Robinhood Instant" for years now
So the "default" RH experience is heavily based on margin but *not! in a transparent way. Deposits from banks are instant under a certain amount (10k for me but it varies) and no T+2