But it's pretty safe to say that those are real violins.
I can only think of two "synthesizers" back then that might have been able to recreate strings, and I doubt Sergio would have used either one. A Moog could possibly emulate strings if given a lot of work towards creating the right sound, but multiple parts would have been recorded on separate tracks, since early Moog synthesizers were monophonic, not polyphonic (multiple notes played at once). It's mind boggling to think that something like Tomita's Snowflakes Are Dancing was recorded one part at a time, layered on each other via multitrack.
The other string substitute would have been a Mellotron, and they did not sound like "real" violins. Listen to "Watcher Of The Skies" by Genesis and you'll see what I mean--those sound "canned." The Mellotron utilized real samples of the sounds by recording them to tape, and using a different loop of tape for each note on the keyboard.
Just the first minute alone is a good example of a Mellotron:
I never thought about that being a little too early for a good string synth, so I'll stick with my statement that that's some pretty fine violin playing in that arrangement. I'll check that Genesis tune out when I get near a computer with good speakers. I guess my favorite music featuring Mellotron is some the Moody Blues stuff.
It's kind of funny how "advanced" they probably thought a Mellotron was back in the day. Here, nearly 50 years later, it just sounds "canned" to me. It certainly does date a recording to hear it these days!
I don't know if anyone has successfully sampled strings to make them sound completely real, but Spectrasonics did a great job with emulating an upright double bass. These two videos give an example of how it works--it even varies the tones slightly (intonation, etc.) to give them a human characteristic.