[this software] + iPad <- Thunderbolt cable -> GPU in external enclousre

You get an AMD GPU, put it in an external case ("enclosure") connect it with usb-c or thunderbolt and that's what's being used in the app. For inference of course

It's not like Apple provides you with kernel APIs for implementing custom GPU drivers on ipad properly.

Which leaves user space drivers embedded in the app as more or less only option. I am a bit surprised Apple allows enough low level thunderbolt access to implement this at all. On a desktop you could implement a user space driver in a shared library so that it can be used by multiple apps, but that's not really an option on iOS either.

Such approach is hardly rare. Many niche hardware products do something similar. Either implement driver/interface for generic data transfer device (like a serial port), or use the slots for vendor specific data within HID, bluetooth interfaces and implement a custom protocol on top of that. Almost any bluetooth device which requires vendor app for configuration, many smartwatches, gamer keyboards/mice which require custom app for configuring RGB lights (often HID based), even MCU programmers and other serious electronics development tools often show up as USB serial ports.

For people who prefer a touchscreen input over a laptop, and if they'd want to do agentic work, or perhaps text to speech, image generation, music production, fancy spellcheck while traveling?