DIY cat flap
“Make do and mend” was a popular government phrase during the Second World War in the UK. Resources were scarce, so you had to use what you could find, often repurposing items in odd places. Perhaps that is what is going on here, a thrifty mindset combined with ingenuity.
Alternatively, the landlord just couldn’t be bothered to fit the cat flap that he was asked about properly. Granted, this hole/pizza box combo does enable the cat to climb in and out of the door, but I’m not sure how secure it makes things, and I’m a little worried about what is going to happen when it rains.