At best, this would take some market share from the low-end of Android phones. Apple's market is at the high end, so I doubt Apple would even notice it.
A significant chunk of iOS's marketshare is people buying iPhones because they are "cool" as opposed to any technical reasons. If Facebook makes their phone seem more "cool" through marketing, that chunk of customers will happily switch.
Instagram is cool and all the "cool" kids still flock to it (though TikTok is significantly gaining popularity). Whatsapp is a Facebook brand and yet doesn't raise any red flags with most people so that's another brand they could use.