Hmmm...

He dated your twin, but claims not to know you were related...

Prosopagnosia only occurs in about 3% of people.

If he is one of those people, then it's possible, however the odds are much higher (in my opinion) that he DID see a familiarity.....

Does it matter? I tend to allow a certain amount of BS in the very early days of relationships. So I take that into account when speaking with someone new....THIS kind of BS, I wouldn't like....
But that's just me.