I don't remember this exact scene in that movie.

However it is true that Barrage balloons were up relatively quickly while effective AA guns took a long time to produce. Barrage balloons were a tactic from WWI that was ready to go quite quickly.

There was a relatively long period of WWII "The Phony War" between Sept 1939 and May 1940 when air defenses were installed but not really tested since Britain, France and Germany refrained from heavy air combat in the west.

In general, Barrage Balloons did not cause many German losses. However, they did force aircraft to fly at higher altitude which increased bombing dispersal and reduced accuracy, When bombers could release bombs at low altitudes (under 5000 ft) they got tighter groups of high explosives and incendiaries, which tended to produce more difficult to control fires that caused much more extensive damage to cities before being put out or burning out.

Barrage Balloons weren't exactly worthless, but they were not the kind of thing that imposed unacceptable losses and ended bomber campaigns. Fighter aircraft were about the only WWII air defense method that could truly stop a bombing campaign.

Source: Ovary, Richard. "The Bombing War: Europe 1939-45"