I’ve had the same problem, as have many others. Unfortunately, there isnt a lot of information online about this bogus popup. It’s not web-site specific, it has appeared for me on many different sites.
For me the only solution was to turn to a third-party content blocker. Support for third-party content blockers was a new feature Apple added to Safari beginning with iOS 9. I chose 1Blocker, which is available in the App Store, but there are other options available. By turning on 1Blocker’s options for blocking ads and trackers I was able to successfully eliminate the bogus pop up you’re describing. Note: I am in no way affiliated with 1Blocker or any software company.
I can appreciate that using third-party content blockers on your device adds some additional complexity to your system. There may be some web sites that are adversely affected by programs like 1Blocker. Fortunately, you can add sites you trust to the program’s “whitelist” feature, which effectively turns blocking off just for those trusted sites.
Despite the added complexity, Apple clearly recognized the value of content blockers when it added the ability to support them to iOS. I’ve been very happy browsing with 1Blocker.
Good luck and let me know if I can help.
— Robert