How can the Teensy Board use avr?

Hey!
I bought myself a Teensy 4.0 Development board.
My goal is to write some bare-metal drivers in Rust for it.

But, I have seen that you can use the avr-gcc compiler for it. (https://www.pjrc.com/teensy/gcc.html)
So, the board uses an ARM Cortex-M7 processor
How can this work?
I mean these are completely different architectures, or am I missing something?

Best regards

The developers of Teensy have made the Arduino environment seamlessly recognize their devices by devising their own libraries and drivers. It does require a special loader-app, "Teensyduino," but works brilliantly.

Ah, okay. Thank you.
It just confused me a lot.
For Rust I'm not gonna need this anyway ^^