The excuse if I remember rightly is that the movie begins at the end of said Italian job with the betrayal. So the title refers to that off screen job as the event that sets the film in motion.

But also, yeah it's pretty much as stupid minis with their trunks loaded with gold landing almost on their front bumper after jumping out of a sewer.

You were only supposed to blow the bloody doors off.

An "Italian Job" doesn't refer to the actual country, it's just the name for a sex act. In the same vein as "greek" translating to "anal"