That's because in the time since Xi took over China there's been 7 different UK Prime Ministers, most of whom were kicked out of office before their term ended.
In that same period Xi has changed the rules so that he can hold on to power indefinitely.
Why are we using British PM count like it means anything. Meanwhile, Angela Merkel served longer as Kanzler than Xi has served as President. Is Germany a dictatorship?
(Neither Britain nor Germany have term limits for their head of government, so China removing its term limit is just bringing it at par with the two most prominent European democracies)