Trump's use of Twitter is a formidable weapon. At home and on the world stage. This is his genius -- his evil genius. How to manipulate public opinion. He first started it with the New York tabloids in the '80s when he used their front pages to push his agendas. Now, decades later, he uses social media like a puppeteer, convincing various factions to dance to his wants.
If  May didn't call him out on the Tweet, Brits might say she was intimidated by him, or that maybe Trump is correct.
Call the bully out, that's the only way.