The link between INT/MND and Accuracy of (Black/White) Enfeebles was suspected from when people first played RDM. Initially there was no Enfeebling Skill Gear (the first piece that was introduced into the game was the Warlock's Tabard), and players loaded up on INT/MND gear so that they could land enfeebles more reliably.
JP need to know how things work so they did tests on how INT/MND affects accuracy. The problem is that Accuracy for Enfeebles is really difficult to measure, as you can't measure it directly. Therefore you need a huge number of samples to reduce the standard deviation (variation) to a point where you can actually tell whether two sets of results are statistically different. Tests were done and a relationship between INT/MND and the accuracy of enfeebles was shown. I'd guess most of it is lost in the past though. Once someone goes to the trouble to perform thousands of tests, the community will generally accept the results and move on, rather than repeating evey couple of months.
http://vnboards.ign.com/the_red_mage/b22541/65019390/p1/?48
This thread (from Vault, one of the original "serious" boards for NA RDMs) has some of the discussion of one of those JP tests. The JP data that was used is long gone, so if you don't believe that there's any effect on accuracy from INT/MND, then this thread is unlikely to convince you. It was the first place I actually saw numbers and formulae for Enfeebles, and it was the first time the NA community and there was a huge amount of data on the referred JP site that backed it up. (This thread marks another milestone too, as the first time NAs found there was a 30% cap on Slow and that the amount Slowed at Casters MND = mob MND ws ~14% although it was incorrectly interpreted as a "Floor" for the effect).