Why didn't they use the ATmega 8u2 with native USB? The small version is no bigger than the chip on this trinket. Eating 3 kB for the tinyUSB stack is annoying.

Also, they claim is has "hardware I2C" which is stretching the truth IMO. The USI doesn't let you write a fully interrupt driven I2C slave, it is a polled implementation leaving lots of support logic to software.