I think quite some people heard of the concept of different kinds of infinity, but don’t know much about how these are defined. That’s why this meme should be inverted, as thinking the infinities described here are the same size is the intuitive answer when you either know nothing or quite something about the definition whereas knowing just a little bit can easily lead you to the wrong answer.
As the described in the wikipedia article in the top level comment, the thing that matters is whether you can construct a mapping (or more precisely, a bijection) from one set to the other. If so, the sets/infinities are of the same “size”.
Not trying to defend the decision, but as far as I know, the reasoning is that if you charge significantly less than you could, it might be because you have other undisclosed agreements with the tenants, like them doing some extra chores for you, repairing the flat or something else. This way, you could avoid a lot of taxes. The sentence also doesn’t seem to be a fine in the narrow sense, but rather a demand of additional taxes. If I’m not mistaken, it’s perfectly legal to charge very little for a flat, but you still have to pay taxes as if you would have rented it out for a regular price.