Very easy, with a catch
Five hours after block 7, with block 6 and block 7 both still open, the author went back to 7 mBTC and called the question very easy: "How many mbtc do I plan to offer as the prize for block 77?" Block 3 already ran on 777 and block 7 paid 77 mBTC, so nobody had to think long. The answer is 777.
That part "would be claimed in milliseconds", so the chain part changes. Not the tail of block 7 this time, but the six characters before the last three of the block 6 key, the block nobody had cracked yet. Six characters to make brute force impractical, and whoever solved block 6 gets this 7 mBTC on top. The author's point: with a chain you can raise the prize on a hard block after it's posted.
The block 6 key ends in AvhDciMQz, so the string is 777AvhDci. SHA-256, entropy, m/44'/0'/0'/0/0. Sixteen minutes after the post the author just gave the answer away: 777, plus the last three characters of this key, HwX, for block 9. It didn't matter. Without block 6 the answer was useless.
Two blocks, one sweep
Funded at 07:41:07 UTC on April 8, fourteen minutes before the post, and claimed at 11:26:31 for 666,438 satoshis. That's the same block as the block 6 claim, and both paid the same address, 1CzEq8yNgX2ax8KQ4T1ocEfQKjrZHePr3y. So the plan worked: one player cracked block 6 and collected both prizes. A commenter wrote "PonkyPink cracked this one as well". The chain ties the two claims to one address, not to a name, so the record says solved and names nobody.