I don't think that's a valid comparison in this case. You aren't advertising another place to buy the same item, you are telling people how to buy content for the item they obtained from Apple's App Store that is (a) necessary for the item to work meaningfully and (b) is not sold through Apple's App Store. Are you completely confident that if Walmart, for some reason, sold video game consoles but didn't sell the actual games for the console, they would maintain an iron-clad prohibition against telling their own customers where those games are available?

Can you show up at Costco and prevent them from selling your products at a lower price than you do on your official website?

Is Walmart the only store you're allowed to shop at?

Neither is the iPhone the only phone you are allowed to buy.