I don't think I ever said that but I will defend those who did anyway. Djokovic became a far better player recently which obviously makes him a bigger threat and he did it with superior grinding qualities, his attacking groundies and bgi serve where already there and haven't changed. Without the better defence Djokovic wasn't anything like as big a threat to Federer.
Furthermore, AO '07 was Federer hitting his peak and it was not easy to predict he would not keep that level up whereas actually he has declined a fair bit (though he is still a great player) since then. Djokovic defence is similar to Hewitt's not Nadal's and Federer already showed that when on form he eats up that play with ease, obviously he is not quite that player anymore. He is more vulnerable and Djokovic is a very good player indeed.
Indeed, it is hard to see a player without great defensive skills, beating Federer. That's why players who rely solely on hard hitting rarely beating Federer. I agree that Djokovic's defense is somewhat similar to Hewitt's, but his attacking play is far better than Hewitt's, which is a key here. It's tough to beat Federer simply by keeping the ball in play. Nadal has given Federer so much trouble by keeping the ball in play AND attacking his weakness.