On the other hand, if ever your case of debate ammo contains citations that run contrary to anything I say, fire.
If I'm reading that correctly, only part of it is askew, actually: the assumption that Count Ganelon was himself Coerthan Proto-Ishgardian.
If Count Ganelon taught "the first red mages", then he should have been from amongst the Elezen of Mhach and Amdapor who fled into Gyr Abania during the Sixth Umbral Calamity, right? After the floods of the Sixth Umbral receded, the Elezen were the first to return to Eorzea (many returning from Ilsabard). 350 years later, the first great wave of Hyur would arrive, forcing these Elezen deeper into the continent. One such band of displaced Elezen were those that arrived in Coerthas and waged a turf war with the dragons before a 200 year period of cooperation later scrubbed from Ishgardian history. It was during this period (350 to 550 Sixth Astral) that Guespiere was forged.
So, if I'm reading that right, first the Elezen became the masters of the rapier. Then, eventually, Elezen would end up in Mhach and Amdapor. Then, during the fall of the Fifth Astral Era, such Elezen would end up in Gyr Abania - Count Ganelon among them. Then other Elezen - still masters of the rapier - would move to Coerthas, where one would forge Guespiere. Thus, there are similarities between red mage weapons and some Proto-Ishgardian weapons - not because Count Ganelon was a Proto-Ishgardian, but because both the Proto-Ishgardians and Count Ganelon were descendants of earlier Elezen rapier-wielders. (Hence the line, "For two groups that are otherwise unrelated, such historical context readily explains the odd similarity between red mage and olden-day Coerthas weaponry.")
Then the Dragonsong War purged any notion of wielding rapiers in Ishgard proper.
An aside, it makes you wonder how Ganelon got the title "Count", right? I even made sure that was the intended title. (It was.)