Any normal politician, seeing his country crushed by a pandemic, would simply extend the Brexit deadline. Boris Johnson, however, is not a normal politician. Brexit and chaos are his brand. As a result, he is threatening to renege on the previous deal with the EU over Northern Ireland; in true Trumpian style, he views potential violations of existing agreements as a form of leverage in the current negotiations.
The EU isn’t likely to fall for it. Tearing up deals will destroy the UK’s credibility as a negotiating partner as long as Boris is PM. He can’t afford that in the long run, regardless of what advantages it may give him in the short run.
It is noteworthy that several prominent members of the Conservative Party have expressed their outrage over this tactic. Would any Republicans do the same? You know the answer to that question.