Not happened lol. If this magical bruteforce program was initiated by this master programmer that would be millions of incorrect logins. These logins would be noticed and this "bruteforcer" would be patched. Multi billion £ companies do not let some delinquent script kiddy into the website bruteforcing accounts and it go unnoticed. If this were the case there'd be a lot more than this 1 kid claiming he got hacked, falador massacre would return once again.
Of course digital data isn't safe, but he did not get cracked that's the more pertinent point to understand.
"and not all data breaches are spotted, meaning someone may have access to runescape accounts or access to data related to runescape that has not been spotted yet."
Again, extremely unlikely, this would not go unnoticed. Jagex would see accounts were getting logged into with either thousands of invalid login attempts or getting into the account using the "data breach". Jagex would be duly alarmed. Yes companies get hacked, i.e the Uber breach. Hackers infiltrated the platform, stole data, paid by Uber to delete the data and told to keep schtum. Companies rarely miss when their data is stolen. The hacking paradigm is almost always caught as it would be at Jagex. This is not a case of bruteforcing. Whoever got this guy either ratted him, phished, targetted him and found passwords in a data breach and hit the jackpot. This guy wants to stay ultimately safe? Random gen passwords, password manager, 2FA on every single account, never buy accounts, don't download programs that brings doubt on legitimacy, always check the url and don't share passwords. With the above being followed you can pretty much guarantee your safe and wont have to post another threat like this again. Be safe brother