But she was wrong to call it absurd. A simple "no" would have sufficed. Countries have bought and sold territory throughout all of history; hell, like half of our country was straight up purchased.
It was 100% a dumb idea by Trump though, agreed. Dumb, but not absurd.
Countries did a lot of things back when they bought and sold territory. The existence of a kind of transaction in the past doesn't make it non-absurd to suggest it now.
Greenland is an autonomous part of Denmark. Denmark <I>can't</I> sell Greenland, coz the Constitution gives the right of self determination to the people of Greenland. Greenland could demand independence from Denmark, then offer it's lands to the USA in exchange for money. But that's not what Trump asked for.
His proposal was unreasonable, illogical, and inappropriate. Which Google tells me qualifies it for being called "absurd".
And let's not kid ourselves. The response would probably have been worse if she called The proposal dumb, like you are.