You're expecting a very high level of expertise for a tool that so many use incidentally. Most don't actually know how transaction isolation works, especially as the fine detail varies between databases. It's incredibly common to believe that databases actually are ACID by default and that is repeated far and wide.
For the last, I was pointing out that people don't necessarily know when they hit race conditions or other transaction isolation faults; they're one of the hardest things to debug or notice. It would be easy to believe that a transaction that confirms a file update would not take a user over their quota and then updates the file would, per ACID, actually ensure the user doesn't exceed their quota. But the default transaction isolation levels don't provide that.
You're actually right because casting my mind back, I can remember exactly the same kind of thing happening in places I worked for. I specifically remember discovering a race condition because they didn't understand about locking/trans iso levels.
Nonetheless if you're going to base your company's product around a database or anything else for that matter, you really had better get yourself sorted out WRT your tools, or it can become horribly expensive to fix or even fatal to your company.
For the last, I was pointing out that people don't necessarily know when they hit race conditions or other transaction isolation faults; they're one of the hardest things to debug or notice. It would be easy to believe that a transaction that confirms a file update would not take a user over their quota and then updates the file would, per ACID, actually ensure the user doesn't exceed their quota. But the default transaction isolation levels don't provide that.