My experience as a daily user of both is that a touch-driven browser running on a battery and CPU limited device with a small screen where that engine is the on

My experience as a daily user of both is that a touch-driven browser running on a battery and CPU limited device with a small screen where that engine is the only approved engine in the platform feels substantially different as a product than a mouse-driven one on a far less constrained system and where other browser engines are explicitly permitted. I can’t say the extent to which there’s source-level sharing, but from a product angle, I think of them as a different product, same as I think of Outlook Web Access/O365 as different from the fat-client Outlook mail client or Word being a different product from Excel even though they’re commonly sold in a bundle. (I’m not claiming to be definitively correct here, just admitting that I agree in one key area with Apple’s argument.)