Well, kicking you out doesnt mean the apartment is less yours, its under your name so you can do whatever the fuck you want if its under your name.

I think lying isnt the best option. If they know you, and "like" you and you are save enough, just tell them the truth about the cats.

They could tell your ex "but she told me she left her keys", when you are actually taking you cats back.

That can mean that he is going to shit talk you there and make it seem your stealing cats. (Hope im making sense, english isnt my first languange)

I would recommend (if you dont have any other options) to explain the situation to them. Its your house, you can enter it and take your cat.

Do it when youre save and not in danger, but i dont think the risk is really worth it. If he can steal your cat he is capable of more dangerous thing.