I think the idea that the EU is seeking revenge on Britain is too conspiratorial. Donald Tusk doesn't get to make those sorts of decisions anyway, as all of the member states have been kept apprised of the negotiating positions all along and get to ratify the final decision. The problem we have is one of incompatible "red line" issues. My own opinion is that the UK's red lines are by far the most arbitrary, as UK politicians have taken it on themselves to interpret "what the British public voted for", generally to suit their own views.
Apart from the undiplomatic language, Tusk is right in general. How would you feel if you spent two years negotiating a deal only to be told you'd been dealing with the wrong people? That's what Tories told the House of Commons last week. There are a subset of Tories who completely misread the EU position from day one, and trumpeted the fact that they were going to get all the benefits of EU membership without any of the obligations. A week ago when the Commons voted on amendments to the withdrawal agreement, some of the same MPs said that if Britain only threatened to walk away from a deal the EU would have to come to its senses. They are truly delusional.
Whether or not the GFA contains anything specific about the nature of the border in Ireland, it's entirely legitimate for the border to be a included in the negotiations with the UK. It's not like some huge concession is being demanded, it's only the transparent border that the Brits have stated they are committed to anyway. There are just two problems: that statement is incompatible with other British red lines, and the Brits have now proved they are in no position to negotiate anything in good faith with the EU due to their own divisions.
The ball is firmly in the UK court.