That’s an interesting take considering Apple demonstrated the quality of the screen by showing it playing For All Mankind. Regardless, we’re all entitled to our own opinion and don’t really get to say what other people should or shouldn’t do with their own money and the devices they buy with it.
That said, an iPad mini screen is 176mm wide, so a 21:9 video would be 75mm tall. An iPhone 13 Pro Max screen is 71mm tall when held in landscape. So it’s difficult to argue that an iPad mini is suitable for watching a movie but an iPhone 13 Pro Max is not - unless the 4mm height and slight cropping at the sides makes all the difference for you. In which case fair enough, but for me the only thing ruining it is the notch/island.
Personally, I don’t want to have to buy, carry, charge, and keep up to date a whole additional (and unpocketable) device just to have a video watching experience that I find acceptable. Especially when phones such as the ZTE Axon 40 Ultra exist, and demonstrate what can be done if we let go of this obsession with face unlock and perfect selfies.