In this case, if the missile was approaching from directly below (or above) and fused at the side of the cockpit, then, yes, that looks a bit late.
On the other hand, if it is approaching from the front, maybe slightly to one side, it could be exactly as designed/expected. Depends on which part of the a/c the fuse will "see". It may be the wing box and/or engine fan that is the biggest return or centre of it. Fuse distance seems most often quoted as 17m, although I have seen other numbers (large radius zone, this is a big warhead). By my reckoning, approaching from front slightly to one side, 17m from wing box / engine on a 777 would put you right about the cockpit window..."
The proximity fuse in a missile like Buk is "side looking" rather, than "front looking" (probably at an angle of about 60deg from missile axis). It is designed to detonate when it passes close to target, there is no point to detonate ahead of a typical aircraft target. The speed of shrapnels from the warhead is much higher than closure speed of the missile/target, so the warhead doesn't work like a "shotgun shot". It just explodes passing close to target and the shrapnels are thrown to the sides fast enough to hit it (shrapnel is several times faster than the missile)
So if the missile was going head-on horizontally or from above with slight side offset, then the warhead detonated as soon as the cockpit came into FOV of side-looking proximity fuse antennas. And looking at the cockpit damage, it was only few meters, so most of the shrapnel was absorbed by front part of the hull with very high fragment density. The spread pattern of warhead shrapnels is not omnidirectional, almost all of the fragments are directed to the sides. Nearly all other plane parts were outside of the shrapnel kill-zone, only one wing and it's engine could get some.
Detonating a bit ahead of target would be good only against really fast target kile ballistic missile or anti-radiation missile, where speed of target/missile is comparable to speed of shrapnels from warhead.