It is a way of speaking. As for the backyard comment was more Greenland Vs. the last isles we sold to the US. Even so, doesn't change the fact that Trump was told no by Greenland, and then has the absurd thought that he could just go to Denmark and ask.
Greenland is not one of our 'states', unlike your states who has to follow POTUS, Greenland doesn't have to follow the Danish government, they wish to follow the Danish Kingdom. So, if a sale was to be done, the US would need to talk to Greenland - Greenland said no (I guess because of the mess they still haven't cleaned up).