An anti ship missile is just a cruise missile specifically designed to target ships.

Harpoon anti ship missiles can for instance be programmed to follow a route then look for a target when they get there and then attack it if it matches certain parameters, that route can be over land. Like any ground hugging missile both cruise and anti ship missiles need to make sure they don't run into the ground so that's already taken care of. Then it comes down to instead of targeting a ship with the missiles internal radar you tell it to target something else when it gets to the area you sent it too. Since you'd use GPS to program it's route I wonder if maybe the anti ship missiles Ukraine has developed are accurate enough just using their GPS that they can target things on the ground using that. Picking out individually targets on the ground using radar would probably be harder than picking out a target against the water but maybe they can even program it to look for certain buildings or an airplane on a runway, that might be beyond it's capability though.

It's all about what the hardware can do and what the software you've written allows you do to with that hardware. The warhead of an anti ship missile might not be ideal to attack other targets but any explosion against soft targets like buildings, planes or ammo and fuel depots is still going to be fairly effective if the strike is accurate.

It's hard to know exactly what they are using for these strikes, hell at one point Ukraine flew helicopters into Russia to attack a fuel depot, but if you keep in mind you might want to use your anti ship missile as a land attack cruise missile while you're developing it a lot of the capabilities you need overlap. It's very possible that Ukraine just has made a missile versatile enough to attack targets on water as well as on land.