That statement is foolish and unbelievable. The S-200 started life in the early 1960s. The S-300 in the late 1990s. You mean to tell me the Iranians were able to leapfrog 30+ years of missile development in a few months? (since the S-300 deal went sour)
Never mind the obvious computer and radar system advancements...consider propellant chemistry and composition advances, metallurgy advances (motor casings), materials science advances (rocket motor throat), etc. etc.
The S-300 is a very different system than the S-200. It's not like hot-rodding a car for better performance. You basically have to start over from scratch. That's not so good. Unless they have had a very extensive testing program they are prone to making all the same mistakes we (anyone who has developed high performance missiles) have made. They're also likely to have made a lot of mistakes in the command and control, radar software, etc. (just like us). There is a definite learning curve to "getting it right" and getting it right the first time on a home-grown system is very, very unlikely.
Probably just a cover story for when the real S-300 missiles being secretly supplied by the Russians are discovered by Israeli intelligence.
FYI; S-200 is known by its NATO name of SA-5 “Griffon
In fact S-300 is a mid’70s design. Still too much a challenge for a nation like Iran is.