So you don't need to use gpiozero or any GPIO library, there's a device tree overlay that you enable and this causes the buttons to type the letters u, d, l, r (top quad), and a, b (bottom pair) when pressed. Since the Sense HAT joystick is mapped to the cursor keys and Enter (for the middle click) this allows you to use something like Pygame or Evdev to read input from both at the same time.
Check out the "Test the buttons" step on this resource: https://projects.raspberrypi.org/en/pro ... light-case
The code in the pygame_test.py example shows how to do both buttons and joystick at the same time.
The Astro Pi FAQ (https://astro-pi.org/missions/space-lab/faq/
) also suggests not to use any GPIO library because a device tree overlay like this will already be hogging the edge detection events for those GPIO lines on the flight units on board the ISS. So if you do include gpio code in your entry it wouldn't run on a flight unit, it would just throw a runtime error.
Good luck with it!