I was dazzled to seek whether it was an internal error or the external factor that made the hack happen, I guess the things only some users are affected made me think it is a kind of phishing attack.
As far as my understanding it is indeed that some users are signing a message outside the original site, some may get the phishing site from an email, based on a few tweets that i saw on the referenced link. The things that the attacker contract was built many days ago, tell it a targeted attack that mainly deceive those who lack security understanding.
I believe their CTO's statements are decent enough to take a consideration:
6) Education on not sharing seed phrases or submitting unknown transactions has become more widespread in our space. However, signing off-chain messages requires equal consideration.
this will be a lesson for those who really don't know much about security protocols. because a lot of them for sure who has items in OpenSea are not really very familiar with the schemes of hackers. so if those users press untrusted links not coming from opensea itself, they will indeed be vulnerable to hacking depending on the malware these hackers imbedded on their links..
also, a lot of them are also new in this blockchain tech market, so they are not aware of some tactics of these hackers.
but wondering if they lost those NFT collection, can these hackers sell it to the market? can they prove that they are the real owners of the items?