I’m pretty sure that they absolutely know that banks make mistakes.
But if you’ve never been on the other side providing customer service, you may be astonished at the systemic fraud/illegal attempts happening constantly.
When I did CS at Blizzard for WoW at least half our work was dealing with compromised accounts (essentially by gold sellers), and then a good deal of “I’ve been hacked!” tickets were by the thieves trying to double steal from an account.
To put that in perspective, it was $100,000s or $1,000,000s in CS costs per month (at just Blizzard) to help customers who were victims of a large scale full time industry (not individuals, organisations) which compromises accounts to make a living.
People banned for confirmed cheating with 3rd party programs would make support requests repeatedly in the hope they’d come across an agent who “makes a mistake and accidentally unbans them”, occasionally you would see lots of them trying the same strategy or telling the same story since they saw a forum post of someone who said “this worked for me”.
Legitimate mistakes happen, but they’re like 1% of the time, the rest of the time it’s systemic attempted fraud.
So the person you’re replying to is most likely aware of this reality and says that “complaining on forums” is most often by people who aren’t telling the full story.
Hence them saying, if your story is legit, take it to the right place to get help, otherwise you’re looking like a just another fraudster (who do this systematically) trying to game the system in some way.