It's not exactly specific to the image preview code, but rather the code that handles the notification when receiving an iMessage.

The attack mentioned in the Wired article[1] relies on iMessage asking the sandboxless Springboard[2][3] to deserialize a maliciously crafted field, included in the incoming iMessage, to escape the sandbox. This specific vulnerability doesn't appear to apply to other apps.

[1] https://googleprojectzero.blogspot.com/2019/08/the-fully-rem... [2] https://en.wikipedia.org/wiki/SpringBoard [3] https://iphonedev.wiki/index.php/SpringBoard

It might be, it’s just that using Safari would turn a zero-click attack into a one-click one (click on my shady link). (Also, WebKit runs with a different sandbox that may require special effort to escape out of.)