There is a code of diplomacy between nations, Bob. If you're about to declare war, then political leaders can send insulting tweets to other countries and call foreigners liars. But if you want a future relationship, abide by normal practices of civility among diplomats.
The tetchy responses that came from the EU do seem to be as a result of irritation at having to deal with Brexit, when the EU has so many other problems, particularly illegal immigration and the rise of the far right all across Europe. They see the UK as bailing out, even though the problems have to a substantial extent, been created by the EU itself by weak, unpragmatic and over-liberal policies.