To add some context, the German constitution states that the German military may not operate within German borders (with some exceptions, e.g. disaster relief during natural disasters). Seeing armed soldiers would be very out of the ordinary.
I do not mean soldiers sensu stricto (German army) but Bundepolizei with heavy weaponry.
Yes, I am sure inasmuch as I have heard this vividly recalled by relatives who frequently visited Germany in the 1970s. But if my anecdotal account isn’t enough, do a Google search or Google Books search for e.g. "frankfurt airport machine gun" and you will see this made an impression on other foreign visitors to Germany in years and decades past.