Hmm. I just have an ordinary Amex Gold Rewards Plus card - I don't think warranty extension coverage differs between cards (but then again I don't have Platinum or Black).
Before I bought the iMac, I called an Amex rep and specifically asked about AppleCare and when the extension would apply - I explained the situation. She specifically said it would apply to the fourth year.
Now, of course phone reps are wrong all the time, so who knows. I guess I should have double checked with another rep.
So, you got me, you could be right - and I've lived with a false sense of security all this time!
I should probably call again at some point...
UPDATE!
OK, so I called AmEx again. I got transferred to their extended warranty department. I talked to the rep and carefully outlined the situation. The rep (Scott, called at 4:55 PM PST), assured me that I am covered till February 2014, i.e. that AmEx is extending the warranty an extra year beyond what AppleCare covers. Now, here is where it gets interesting. He claims that however, I should be clear that what is covered by that extra year is NOT AN EXTENSION OF APPLECARE, but an extension of what basic coverage would be - in other words, whatever Apple provides in the first year if you DON'T buy AppleCare, so the fourth year here would be just like the basic non-AppleCare coverage from Apple would be. I guess that means certain niceties are out, such as phone support(?) and home visit by technician etc.
Hopefully this information is correct. But hey if a rep in the extended warranty department is wrong, what can I do. Or as my wife says: "you'll find out if you are covered or not only when you need it and file a claim"... that's when you find out that no matter what the rep said on the phone, guess what, you're out of luck. Reminds me of certain medical insurance experiences - everything is great and you feel fully covered and so you pay your premiums for years and years, and then one day you submit a claim and all of a sudden you find out that - oops, bad luck, you are not covered for this particular thing, hee-hee, sucks to be you!