This would hurt the EU as well because:

• those t bonds serve as a reserve giving the euro value as it can be exchanged for USD, if they sell the t bonds then people can’t exchange them for USD as easily and the value will go down as it’s harder to buy things outside of the EU which is usually denominated in USD, and a currency is only as valuable as what you can buy with it.

• this will crash the US economy and every recession that the US goes through Europe ends up getting dragged into as well

• dumping that many t bonds would reduce the price and thus they would have to sell them at a loss, leaving multi hundred billion euro hole in there balance sheet

This isn’t to say they can’t do it, just that it would cost them a lot and it remains a question if your average EU citizen would tolerate these hardships for Greenland. They’ve already shown they aren’t willing to completely cut themselves off from Russian gas for Ukraine, and Ukraine seems a more relevant cause for them then Greenland.