I agree with you that Apple could not really have feasibly avoided releasing iOS4 on the 3G/3GS, but the author makes a fair point that they do own the whole stack, and either didn't test performance or didn't care enough to tell people that it would perform slower on older hardware. It's not like iOS 4 on 3GS hardware was an unforeseen combination. From what I've seen from friends and on Youtube, the problem isn't minute, either - it's a drastically worse experience. It seems like something like that should have been caught.