I wouldn't say that this is hacking or that the owners of those wallet was careless with their private keys. It was an intentional act by Zhank because he wanted to steal their funds that was why he made a back door to have access to people's private keys.
This is why open source wallets are the best wallets for storing bitcoin because it is open for all to look into the source code and improve the security. It is only a closed source wallet that a back door can be created unknown to the users. An open source wallet can not be compromised only if you expose your private keys or malware and Spyware attack your system unknown to you, because there is no back door.
They were mistaken in trusting the use of the Houbi wallet or any wallet managed by centralized authorities. Indeed, since the wallet was a closed source, this is what made the unethical employees succeed in this heinous fraud by placing the backdoor in it, which gave them unauthorized access to its users' seed phrases and private keys. I agree with you that with open-source wallets, security vulnerabilities can be examined, discovered, and fixed, and they receive continuous updates; the mistake that may expose the user of these wallets to hacking is downloading fake wallets from unknown sources.
The fact that they have now closed this wallet and launched iToken might be better. Still, I personally only trust well-known, open-source Bitcoin wallets recognized by the crypto community, avoiding any other untrusted wallets.