I think he's wrong! My GC clearly shows Turkey on it, complete with the address of the relevant organisation there. It's a long time I bought my GC in TR, but I remember it being very cheap and it covers you all over Europe, which is probably what you want.
In fact, ALL GCs show TR and all other countries participating in the scheme (including Iraq!), but some insurance co's cross out the more "exotic" countries.
So, if they don't make you take out a policy at the Iranian border try to get a TR policy asap. It's not a good idea to ride without...
Salut from Southern France, the bikers' paradise,