From what I know that's a shortcut for your phone desktop, but there is no need to install any software.
That's the feature of some mobile browsers today but yes, there should be no pop-up. Although at the screenshot provided, seems to be legit but it's not necessary at all to click the "install" as the site is just 1 click away.
Not checking the site yet, so can we use our NitrogenSports credentials here to log in, or need a separate account?
Anyhow, I can check that later once got home.