Exactly. You have to take into account that Achilles is an old man at that point, and when he sees so much potential in Connor, he realizes that his sort of "withdrawl" from the Assassin's was a mistake, and sees an opening to rectify it.
It's kind of like the Luke/Obi Wan relationship. Their order has died down quite a bit, which makes the need for tradition a little less pertinent. I think Achilles even mentions when he's inducting Connor into the Assassins that "we used to have a ceremony for this kind of thing" but that neither of them are the type to worry about that stuff. Something along those lines.
Also, the passage of time is always a bit vague in these games, but I got the impression that at the time of Connor receiving the robes, some considerable amount of time had passed already anyway. Much of it was just skipped over, so as not to spend too long on Connor's training.