Surely the simplest explanation would be that it was a mistake or an error in judgement?
Your proposition would suggest that Apple deliberately withheld a feature that they had (perhaps unintentionally) advertised as being available for all iPads. Why? I for one have been using multi-tasking gestures on my iPad 1 with iOS 5 for some time now without any problems. All it took was a couple of lines in a .plist file.
You are also forgetting prior evidence of a reversal of policy in iOS as a result of user pressure: the case of the disappearing then reappearing "rotation lock" switch. We know for certain that that wasn't planned all along and such evidence must surely be worthy of inclusion in your "simplest explanation" deliberations.
On the balance of the evidence "it was coming all along" would not appear to be the simplest explanation. If anything it would be even more maddening that Apple had remained silent on the issue if that were the correct explanation.
Why would it bother you if it had been a "few noisy users" that had effected the change anyway?