The delay technology did exist in the 70s but it wasn’t always used, so I believe.
Of interest, what storage technology was used to implement the delay? When RAM chips were specified not in gigabits, or even megabits or kilobits, but in bits, it seems unlikely that the storage buffer would have been digital.
Magnetic tape, maybe. But assuming finite media resources that would maybe suggest an infinitely reusable tape loop, was that possible, or reliable?
Curiously the only job on graduation that I didn’t get offered, but desperately wanted, was with BBC Engineering. If only I had got the job, maybe I’d know the answers.