Couldn't someone have just imported the 1HKnuXP3RZDLjHPuDcj4EzaDtEH94FMbgB into there wallet and then used it as an input in there next transaction they were going to make anyway therefore it paying 7740 sat of the 10,00 sat miner fee that they were going to have to pay to send any BTC anyway?
Yes they could. But maybe they wanted to claim it before anyone else did, idk.
Noob question here, but i thought that privkeys always started with a 5, what am i missing here?
"compressed" private keys begin with a K or L,
"uncompressed" private keys begin with a 5.
Check it out by playing about in the 'wallet details' tab on bitaddress.org: