There was a tiny kernel of truth in what the shop was saying, but it was a tiny kernel, and there's no problem for any intelligent mechanic.
Current (since 2000) Campy freehubs use a single circular spring for the ratchet pawls (not the bearings), which also keeps them in place when the hub is disassembled. However on the first generation hubs the 3 pawls each had a tiny coil spring below and nothing to keep them home outside of the ratchet ring. So Campy provided a circular clip that held them in place while you fitted the freehub body into the hub shell. They went halfway together, then the clip was slipped off and you pushed the freehub the rest of the way in.
If you have 1st generation hubs, it's still not a serious problem. First work carefully as you slide the freehub clear of the ratchet ring to make sure you catch any pawl that might spring free. If you're nervous, pull the hub apart in a bucket just in case. For assembly without the clip, wind a few turns of dental floss or fishing line around the pawls to keep them in place, assemble the hub partway leaving a small gap and unwind the floss.
So, yes there was a clip, but no you don't need it, you can use dental floss instead.