From the snopes article you obviously didn't read:

This might seem like an innocuous comic scene if Travers’s novels didn’t associate chimney sweeps’ blackened faces with racial caricature. “Don’t touch me, you black heathen,” a housemaid screams in “Mary Poppins Opens the Door” (1943), as a sweep reaches out his darkened hand. When he tries to approach the cook, she threatens to quit: “If that Hottentot goes into the chimney, I shall go out the door,” she says, using an archaic slur for black South Africans that recurs on page and screen.

The 1964 film replays this racial panic in a farcical key. When the dark figures of the chimney sweeps step in time on a roof, a naval buffoon, Admiral Boom, shouts, “We’re being attacked by Hottentots!” and orders his cannon to be fired at the “cheeky devils" [see below]. We’re in on the joke, such as it is: These aren’t really black Africans; they’re grinning white dancers in blackface. It’s a parody of black menace; it’s even posted on a white nationalist website as evidence of the film’s racial hierarchy. And it’s not only fools like the Admiral who invoke this language. In the 1952 novel “Mary Poppins in the Park,” the nanny herself tells an upset young Michael, “I understand that you’re behaving like a Hottentot.”