Who is Twitter to fact-check world leaders?
When world leaders rarely tell the truth, how can anyone realistically think that such a system could even work, even if it made sense?
I'm going to take you at your word and accept that world leaders rarely tell the truth: so they should ALL get the same treatment then. But instead of stamping their output with just "fact-check this", why not unilaterally label all of it with: "may contain lies, omissions and half-truths"?
Even if Twitter's motive was to help its users, that's just common sense. Does Twitter have such a low opinion of its users that it needs to treat them like 5-year-olds?