How did you try that? I just reviewed the image, and firmware-wise (/boot/config.txt), the onboard WiFi device isn’t disabled, so it should be possible to enable it without reboot. Otherwise every regular WiFi setup on first boot would fail as well.
But you are right that onboard_enable is missing. So while it should succeed in first boot, it would fail after reboot, as then the disabled device setting in /boot/config.txt kicks in. Okay so on first boot, wlan0 is assured to be onboard WiFi, as it is enabled (firmware-wise) and WiFi Hotspot by default uses wlan0. This means at least on first boot, we need to enable onboard WiFi explicitly, when WiFi Hotpost is installed. When this is done after first boot, the (interactive) menu throws an error when no WiFi interface has been found, so then it is okay that users enable it in network settings first. We could even offer that when we know it’s an RPi model with onboard WiFi and see it disabled.
The country code is applied automatically on first boot and applied to /etc/default/crda to be loaded on every next boot via udev rules, as fast as the WiFi device is attached/found by the kernel, but indeed only when WiFi is enabled dietpi.txt wise. Actually it makes sense to do that regardless whether WiFi is enabled or not, so that the correct country code is applied as fast as any WiFi adapter is enabled.