arduino also supports darn near every 8 bit atmel mcu though the use of core files
I'm not sure I'd say that. Arduino supports atmega8, atmega168, atmega328p, atmega1280, and atmega2560.
It is expected to support a couple of others, and various third parties have added and/or otherwise implemented some compatibility with quite a few additional that span a large portion of the AVR "space." But those last have varying degrees of "support", and none that comes directly from the Arduino team.
Not all AVR cpus support a bootloader. But then, the arduino bootloader is not strictly necessary to run Arduino code.