Is this a scam? A web page claims my iPhone is infected
Yes. This is a scam. You are 100% safe to close your safari tab to get rid of this message and absolutely nothing will happen. Many scammers provide fake instructions which will tell you what to do to give them access to your device, or make you give them money or personal details too, without you realising.

In this case, due to the web address, it appears the only intention of this fake site is to get you to install an app, probably to make the app seem more popular. The address implies this is an advertisement of an app, and the bit we can see of the instructions seems to back this up too. It is possible they are still trying to get money if the app costs money.