We're not talking about a Titan. We're talking THE Batman. Any rational thinking hero who knows Batman wouldn't immediately assume he is dead. It makes sense with the Green Arrow, Barry Allen, Hal Jordan, or those Titans nobody cares enough about to bring back because they don't have the same reputation of escaping death and impossible situations.
And yes, Tim does have to think Bruce is one of the oh-so-rare lucky ones because he's Bruce Wayne. As a member of the superhero community Tim has seen a few less clever heroes crawl back from death, usually with help. In this case Bruce has the advantage of not actually being dead. Realistically there should be a lot more people in the hero community besides Tim and Conner counting down the days until Bruce comes back because if Oliver Queen can do it, Bruce Wayne can do it with no hands.