How was transaction a4bfa8ab6435ae5f25dae9d89e4eb67dfa94283ca751f393c1ddc5a837bbc31b mined?
"Relayed" and "mined" are completely separate processes and should not be conflated.

The transaction is valid, so it was perfectly fine to mine it (i.e. include it in a valid block).  

"Standard" is a more restrictive requirement, and as you say, this transaction is not standard.  That means that a default version of Bitcoin Core will not relay it, i.e. will not transmit it to other nodes on the Bitcoin peer-to-peer network.  But if the transaction reaches a miner some other way, they are perfectly free to include it in a block.

We can only speculate as to how that occurred in this case, but here are some possibilities:


Some people were running a modified version of the Bitcoin client (or alternative Bitcoin software) that does relay non-standard transactions.
The creator of the transaction sent it directly to a miner, by some means other than the peer-to-peer network.  Some miners or pools have web sites where anyone can submit a transaction.
The transaction was created by the miner herself.