Because all the funds from 161 through 255 were redundant (Bitcoin addresses only use 160 bits) the author of the puzzle moved all the funds from the 160-255 ranged down into the 1-160 range. It is explained up thread.
Is it? I thought it uses almost 256 bits (the last possible private key is FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364140)
The maximum value of a Privatekey is the value that you pointed, what they actually want to said is that address is a reduction of the public key, it is a 160 bit hash, so if you are capable to fin a collision on a 160 bits hash, that mean that you are capable to find any collision, regarless that those private keys are up to 256 bits