The reason OpenVPN (and almost all VPN solutions) use such ridiculously long keys is that you don't have to brute-force them. An actual cryptographic attack on the algorithms commonly used for VPNs (or for HTTPS connections to websites) looks a lot more like a dictionary attack than a brute-force attack; you can discount great swathes of the problem space without having to actually try them.
That explanation seems fine to me. It's not an explanation of the underlying math but it doesn't need to be.
You are probably getting downvoted because the article points out exactly that. The author was erroneously thinking that a big key is necessary - and wireguard doesn't provide one. Which made him discover that fallacy.