Non-technical people don't have a concept of "in app browser sandboxing". In their minds they clicked on an ad, they're still inside TikTok, TikTok's UI is showing, TikTok will show prompts based on the content shown... they probably assume TikTok has access to that page?
Honestly I'm more annoyed that Apple allows big apps to use the loophole that is the legacy webview than I am that TikTok uses that webview to do the exact single thing it's good for... having full control over the web content you're showing in app.