How old is the hack? I used it years ago for half a bitcoin which I hope I still own.
Edit: If I remember right, I used it in 2016 and I opened a private window, disconnected the Wifi, created the wallet, closed the window and reconnected. Meanwhile I think I watched the devtools too. I still wonder if some data got queued in some web workers and was sent via websockets even after closing the private chrome tab. Back then browsers have been less privacy focused.
It seems the hack started in 2018, so you should be safe. Note that all the procedure of incognito tab, disconnecting from the internet, etc., wouldn't have helped you, as the key has been generated on the server before you loaded the page :(.
When I came across the site years ago, I saved a copy of it so I could run it locally in my browser. Only later did I realize there was a GitHub repo: https://github.com/cantonbecker/bitcoinpaperwallet
Edit: If I remember right, I used it in 2016 and I opened a private window, disconnected the Wifi, created the wallet, closed the window and reconnected. Meanwhile I think I watched the devtools too. I still wonder if some data got queued in some web workers and was sent via websockets even after closing the private chrome tab. Back then browsers have been less privacy focused.