Not only is the website owned by a particularly sketchy individual (see the stickied automoderator post), the instructions are only about a “paper wallet” which is one of the worst ways to make a cold wallet.
A paper wallet is essentially a piece of paper with the private key to your funds. You can create this on a completely offline computer, so it never touches the internet. In principle, that’s perfectly fine. However, as soon as you want to spend your funds it becomes complicated, because you’ll have to import your private key from the paper into a software wallet. This not only exposes your funds to the risks of a software wallet, it also makes that paper wallet useless going forward. If you only want to spend part of your funds, you have to repeat the process of creating a secure paper walllet.
The norm for cold wallets these days is the hardware wallet. This is a device that holds a seed, or master private key, that can be used to manage any number of addresses. The hardware wallet is secured with a chip that ensures the seed can never be read by an external party (such as a connected computer). All calculations that require the private keys are done on this secure chip and only the result is sent back to the computer.
There are a number of different brands of hardware wallet, Ledger, Trezor and Coldcard being the ones that are probably the most well known. They’re all solid, but each has different advantages and disadvantages.