While I felt Ferarri really did leave it off till too late... they most likely got it exactly right.
Remember, the strategy relies on the difference in pace at the end of the race, not the average difference in pace. Doesn't matter if you can mathematically catch someone... what matters is how much faster you are when you catch him.
Kimi's advantage, with tires that were six laps fresher than Nico's, was two seconds at the start of his last stint. At the end of that stint, the advantage was down to around one second.
If Kimi had pitted three laps earlier, his advantage over Nico would have been less. He would have caught up sooner, but he'd be stuck with a lower time differential and less chances of passing. And, of course, Nico would still have tires left to counterattack if he overtook early... and if he left it off till the end, he would probably only be half-a-second faster in those closing laps, with little chance of overtaking.
Whenever he pitted, he would not have had the pace left at the end to take Lewis. Yes, Lewis was experiencing the same brake problems Nico had, but by the time Raikkonen would have caught up, (EDIT) his lap time differential to Lewis would have faded away to nothing.