Is it involve risk connect metamask to opensea? hacks? possible?
I think they cannot access your wallet they only allow you to have an account with your wallet that is what they said. But after visiting the site you check that you are on their official real link, check the spellings of the link so that you do not enter on any other fake website.
Other point is that do not enter your metamask details on any website as while opening/connecting to any website you will need to give password or key only on the metmask extension but at that point you have to check before that your PC/device is safe from any virus as there are some viruses which can monitor your keyboard and can copy what you type.
It all depend on you, you always need to keep your device safe from any attack. Plus do not click on any link or linked word, if there is a linked word sent to you then right click and choose "Copy link address" and then paste it somewhere to check that which link is there. If you do not recognize the link then remove it and do not visit there as it may be from hackers.