It's wrong, but probably not for the reasons you expect. 320/120 is a myth. Resists don't magically become less when you hit 320 skill. 120 INT is only meaningful if you're fighting a mob with 110 INT. 320/120 is like a bad edition of "BLM for dummies", and it's time people stop citing it.
When fighting a resistant mob, you need to maximize your equivalent magic accuracy. 1 skill gives magic acc. 1 INT gives 1 magic acc, but only until your int = mob int +10; after that, 1 INT gives 0.5 magic acc. Do not remove skill for an amount of INT that gives less accuracy, unless we're talking about something significant. 8 INT from mahat. pants, for example, is preferred over the 5 skill from druid's slops. In the worst case, you'll lose 1% magic hit rate, but you should gain ~2-3% in raw damage from the INT (plus -enmity is good).
The details of Genie vs. ACP were pretty thoroughly covered in the previous thread, and it's basically a toss-up.
320/120 is not a myth, its a baseline build. People came up with that build with specific monsters in mind (Kings and sky gods, etc) all of which have around 110 int. For monsters with higher int, the amount of int in your resist build becomes even more important.
You're right in that it isn't a "Magic number". People shouldn't get 320/120 and then stop there, but its a good target to shoot for when first starting your resist build. From what I've read of Kanican's blog, it seems like he was getting very few resists on any HNM with a 338/130 build.
In light of the conclusions about magic accuracy and int outlined in Kanican's blog, here are my thoughts about Redingote:
If you don't have Morrigan's robe, this piece is very good for a resist build for mobs that don't require a max int build (aka, kirin, where you would be using errant / mahatma anyway).
Even if your int value is over 10 higher than your target's int, this body has higher magic accuracy than a genie weskit... 5 int * 0.5 + = 2.5 maac + 4 maac = 6.5 maac which is greater than the 6 elemental skill (equivalent to 6 magic acc) on Genie weskit.
For more traditional HNM with the average int values of 110, Redingote comes out on top of Genie weskit with 9 magic acc.
In either situation, Redingote is better for resist build than Genie weskit, and if you have NQ weskit the Redingote shines more in the maac department and will even outdamage the NQ weskit.