All considered that means antpool has a 2.5-3% chance of flipping an wanted result into a wanted result. At $125,000 per block, that's $4 million of cost per bit manipulated favorably, getting exponentially worse as you need to manipulate more bits.
And remember that the timestamp of the block is oply an approximation. You can use any time you want. I'm not sure if the net reject the block when the time is too off, but you can use a fake delayed time of a few minutes and wait to broadcast it (perhaps mining secretly the next block), or perhaps you can use a fake previous time of a few minutes and blame the delay of the broadcast to a bad connection. (It's mode difficult to do this secretly in a pool.)
The newspaper here make a big fuzz about the first baby of the year, and I always suspected the exact born time in that moment is not reliable.
In essence, the difference between Bitcoin and an aggregate scheme is that in Bitcoin, your control over the entropy is probabilistic and directly proportional to the number of bribed parties, whereas in an aggregate scheme, you have no control whatsoever until you manage to bribe every party, at which point you gain full control.