I'd like to know what Gabe meant by fraudulent too. The whole idea of crypto is that if you send it and it's confirmed you can't get it back, which makes fraud (on the payers side) nearly impossible.
You brought up the point about zero confirmations. Zero-conf itself is fairly reliable, it certainly doesn't account for 50% transactions being fraudulent. Bitcoin introduced replace by fee in 2016. In 2017, bitcoin transition fees were high, and there was large increase in the bitcoin mempool, which led to a backlog of stuck low-fee transactions.
Replace by fee adds a major loophole to zeroconf, low-fee, stuck transactions could be rebroadcast with a higher fee and sent somewhere else even after they were "received" but not yet "confirmed" by Steam. Users could sign up, pay with Bitcoin, steam sees the payment but doesn't want the user to wait 3 days for confirmation, so steam completes the signup. After that the user rebroadcasts the transaction back to themselves with a higher fee and quicker confirmation. The original transaction doesn't go through.
Here's and discussion on how reliable zero-conf is when RBF is not part of the protocol and instead bigger blocks are mined.
https://www.youtube.com/watch?v=TIt96gFh4vw