Yes and no--the technology of sampling moved awfully fast. The first commercially-available digital delay and looper was the Eventide HM80, in 1978. All that remained was development of the pitch and triggering controls, which did not take long, since control-voltage pitch and trigger keyboards were already in use with modular synths. Cabaret Voltaire claimed to have had the very first triggered-playback digital sampler; I read that claim around 1984, but i don't know when they first had the device.
So I agree that in 1979 there's almost no way ZZTop could have had foot-triggered digitally-recorded horn sounds. But just a year or two later, it's actually quite plausible.