I think this is a ridiculous thing to argue about. It's like saying "Why does Obi Wan think that Darth Vader killed Anakin Skywalker? Doesn't he know Darth Vader is Anakin?" Of course he does! He is giving Luke the information that he needs to know in the way he needs to hear it to start him on his path to being a Jedi. Hence, the round about talk of Anakin/Vader.
Obi Wan never said "I don't recognize this droid." He said "I don't seem to remember owning a droid". Jedi usually only "owned" what they could carry on their person. The closest droid Obi Wan ever came to "owning" is R4 (seen in episode II, III, and The Clone Wars) but it's definitely possible that R4 belonged to the Jedi order and was just Obi Wan's favorite droid to use.
R2 definitely didn't belong to Obi Wan because he was always used by Anakin and then given to Senator Organa after order 66 and presumably still belongs to Senator Organa since he is being used by his adopted daughter (Princess Leia). As far as R2 saying he belonged to Obi Wan; it's shown that R2 is capable of lying while conducting covert missions. This is why he is so valuable. In effect, the easiest way for a droid to convince a stranger to bring it to the person it needs to go to is to say it's owned by that person.
R2 doesn't immediately recognize Obi Wan because, really, would you? So far there is no indication that the two have interacted since order 66. The man has changed his beard style and aged 20+ years. This last point I have, honestly, doesn't prove anything but when Obi Wan first sees R2 he refers to him as "my little friend". Maybe Obi Wan is just being friendly but MAYBE considering all the things I've pointed out he does, in fact, recognize R2. I'd like to think so.
