They base their conclusion on one opinion survey (purchasing managers’ index) - no economic data.
While the International Monetary Fund has trimmed its world growth estimates (based on data-fed models), this article tries to convince the gullible that only the UK is falling short - aaaand, also make the logical leap that it is caused by the Brexit vote, just as a sort of assumption.
That's what lleftists do. Make up their mind based on feelers, not facts, especially when they're pushing an agenda.