This doesn't happen. People are just applying stereotypes to every possible context and situation. Germans are perfectly capable of coming to a BBQ later than the scheduled start time, although it's true that unless it's impossible for them to make their schedule fit with yours, if you tell them 5, then 5 it is.

Because of this, I would say it's even more important to schedule things sufficiently ahead of time. Making up plans only 2 or 3 days in advance, when weekend plans are mostly settled and they can't properly schedule around your event, will just stress people out.