This is intended, when you run on HW_MODEL 21 (which is x86_64) you get the driver menu instead of the resolution menu, see
https://github.com/MichaIng/DietPi/blob/5f69eebea01ce277d48333ffa21b844d5a068c3b/dietpi/dietpi-config#L118
here is an older thread with a similiar problem and a solution
https://dietpi.com/forum/t/terminal-resolution-on-x86/13828/7