I don't know if it's a mistake, I've come across it on a number of games on itch.io when trying to jump to patreon or discord from a link on their page. I suspect some form of algorithm puts it there for certain pages. It doesn't actually stop me going there, just gives me that warning.
Considering how many people have had their games uploaded to itch.io by someone else and infected with malware, they might just be covering their ass, by saying if you go there and get burnt, don't blame us. (But I'd still message them to ask of course).