In a recent Donbass conflict, there were reports of Ukrainian military and militia using specially tailored Android apps with OSM data as base layers.

If you can get perfect coordinates from Apple Maps and your missiles have GPS, you're good to boom. It's not as though the missiles have an iPhone inside them.

But if your missiles have GPS surely you have satellites to get the coordinates yourself?

I don't think that follows. GPS access is universal; accurate mapping data is not. These rules are an attempt to ensure that precise coordinates for key targets are not available to anyone with a satnav system or smartphone.