Ok, let me repeat you one more time, and take it from someone who traveled HUNDREDS of flights on Swissair's MD11s in all classes. The seats at the left and right hand side of row 18, those are 18 A-B-C and the 2 on the other side, DO NOT feature extra legroom.
The middle section, however, does have extra legroom as this is the first row of Y-class (no row 17 in the middle section). Best seats there are 18D and 18G.
So, whatever your Swissair agent told you, it is not correct. Most probably, he or she has never been in that seat, and, furthermore, telling you what you want to hear is a very easy thing to do.
But, than again, you don't seem to believe me, so I suggest you just check it out yourself!