However, one of Putin's aims is to destroy NATO and the U.S. attacking Greenland would likely lead to that. Also, Trump seems keen to flex military might (despite his FIFA Peace award), so it's not wise to ignore threats that he's made more than once.

It often makes more sense to view Trump's behaviour as him being a Putin plant to destroy the USA and weaken Europe. I'd guess that Putin has a lot of incriminating evidence of Trump being involved in dubious sexual acts.