It's somewhat common approach (I think USB was only done in Macs, in fact). HID over I2C/SPI is somewhat common solution, taking over from PS/2, and of course you need an MCU for the matrix scanner and the touchpad anyway.
the BCM5976 is dedicated touchscreen ASIC that Broadcom makes for Apple for their iPhone and iPad devices, there's probably MCU (or four, this is broadcom we're talking about) inside, but that's probably because it's simpler to implement this way.
Reminded me of the Raspberry Pi Pico W[1], where they paired a dual-core ARM Cortex M0+ MCU with a wifi chip that has both a Cortex M3 core and a Cortex M4 core[2].
The Arduino Unos being paired with an ESP-01 module is similar. Tons of other examples out there as well of course.
Not sure if there's a similar "power inversion" at play here, just find it funny when it shows up.
