He said "Fortunately I managed to keep a few secrets even from him..if I hadn't I'd be dead right now".
That means that he would have been dead at the time of the attack (which technically is in the past, since it happened a moment ago) if he hadn't kept his secrets from Itachi,
Tell me, what secrets could he be keeping a moment ago that saved him?
If he meant about the attack, he would have said something like "Fortunately I managed to PHASE OUT from his amaterasu...if I hadn't I'd be dead right now".
AND he even has this thought in his head, right after he says "I assume he set things up so the sight of my Sharingan would automatically trigger Amaterasu to activate.
He has the thought in his head means he was shocked that itachi was actually planning to kill him even before his death but what prevented him was the secrets he kept. Itachi wasn't sure whether he was madara or not (he told his brother before they battled "madara is a liar", when sasuke asked about him) and he also wasn't aware that tobi led the attack on konoha using the kyuubi. (Infact, everyone that came in contact with tobi, died during the attack except baby naruto and so no one knew who was responsible). I believe these are the secrets he kept from itachi that saved his ass if not, he'd been dead a long time ago.
That's why he made the the statement:
"FORTUNATELY I managed to keep a few secrets EVEN from him..if I hadn't I'd be dead right now".
FORTUNATELY- Having unexpected good fortune; lucky.
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In other words, obito was thanking his stars because he was still alive.
EVEN- this smacks of the fact that itachi was an exceptional top rank anbu spy. Remember he was a double-agent for the leaf and his uchiha clan. So obito wa
s somehow impressed with himself that he didn't let itachi find out about those secrets. That's why he mentioned "managed".
Him meaning if they were to have fought is baseless assumption.