I thought the wallets couldn't be hacked.

He has a 30% success rate. That's pretty high.
I think the rate or was likely to be hacked it depends on the security of your wallet. there are many ways to hack.
any phishing, viruses, and etc. if you do not add security as 2FA in your wallet. it could be possible to get higher hacked. so use the best possible security. if you do not want your bitcoin stolen