As a Dutch I don’t understand this joke
After reading through the comments it could also be related to climate change but what first came to mind was the american housing crisis. Some people seem to think that the housing crisis could be solved by having more room for houses while, how I understand it, it is actually not a problem of available living space but a problem of affordable living space.
A simple map of population density across the US should clearly show that lebensraum is not the problem indeed. It’s more a question of places that are both desirable for the opportunities they offer, while being affordable.
The joke is that we have more land to the north so we don’t have to. We can just invade Canada before we start with polders