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Gmail does the same on iOS. And they also lie about it.

On iOS, it prompts if you want to either download Chrome, or use Safari. If you tap safari, it doesn't open safari, but a webview inside Gmail.

Note that, while the rendering engine is the same, an app can't spy on my navigation on safari, but it _can_ on a webview it spawned.

Gmail on iOS uses SFSafariViewController, which runs in a separate process from Gmail.app and can’t be interacted with, unlike WKWebview.

My guess is that they ran metrics and decided that keeping users in the Gmail app window led to longer engagement than opening Safari..