It doesn't seem to discuss Trump's "offer". Voting independence from Denmark is different from being given the option to join the United States.
As Chomsky would say "whenever there are multiple pictures, the darkest one tends to be closer to the truth". What if natural resources would be more expensive (for both US and EU) to buy if Greenland were independent, than if it were still half-colony of Denmark. Then EU and US would have a common interest in manipulating in the same direction the referendum you referenced (for independence). Both US and EU might have cheaper access to natural resources if the population votes no for independence. Good Cop Bad Cop stuff, to scare the population to stay subjugated (and enjoy imaginary protection from EU against imaginary threat from US).