There is some ancient history here. The bloke at the test centre is talking complete twollocks of course. However if we all put our flared trousers back on and whizzed back to 1975 he would have had a point.
FIAT 'sold' the 124 to the USSR and helped them build a new factory and town, which they named Toggliatti after the founder of the Italian Communist party. This factory produced the Lada, the reworked (AKA ruined) FIAT 124. The USSR being a closed economy had no foreign currency, so they paid the Italians in kind, and part of that payment was steel of rather poor quality. It really was dreadful, and Alfas and Fiats were made of this very thin brittle metal. They were orrible.
The big improvement in build quality probably started with the 75, leaped forwards with the 164, then seemed to take a small step back with the 156/166, but they are certainly NOT made of Russian steel. Yours has rusted for other reasons.