As others have said, I think it's an exaggeration to say that Danny's 100% sure Shaq will be back. However, I also agree with many who agree with the OP's general sentiment that Danny must've been very confident that he was going to have one or both of the O'Neals back to make the Perk deal.
That said, I also want to come back to what I'll call the Myth of KG's Knee. While it certainly seems like Danny lied about the knee (though I suppose we'll never know if KG would've pulled a Willis Reed had we made the Finals), Danny did it for a competitive reason. Thinking that KG could be back not only put fear in opponent's minds, but also wasted prep time at opponent's practices. It also potentially kept the hope alive in the active Celtics, who might've thought that had they eked out a win in Orlando, they might have KG against Cleveland.
With this situation, it's completely different. No regular season opponent is fretting about whether Shaq or JO will be back next time they play us. Furthermore, lying to the Celtics would only hurt the team, as they'd be less likely to worry about size, defensive, and post problems that the current team has, thinking Shaq and/or JO will solve the problem when he/they come back.
I'm not here to defend Danny's honor or convince you that the Perk deal was the right move, but I am here to tell you that I think the notion that Danny's lying about Shaq and JO is absurd.