343 improved input device support with Infinite so now you can use controller and keyboard and mouse at the same time. When using an xbox controller and mouse the game does prioritize the mouse and disabled aim assist, but when you use a Steam Controller with the right touchpad as “mouse joystick” and gyro as “mouse” (mouse joystick emulates a gamepad’s stick), because you hold your thumb on the emulated stick the game thinks you’re using a gamepad for aiming when in reality you’re using a mouse controlled by gyro, giving you mouse-level accuracy and controller aim assist at the same time.
Thankfully the aim assist doesn’t seem to be as strong as it is on MCC (where you barely need to touch the right stick when in range), or maybe the faster paced combat counters it more. If it was as bad as that, it would be impossible to miss.