Another possibility is the stratum/pool mining software responsible for this block is not connected to a full node, rather a lite or SPV node. To build a valid block, you really only need to know the previous block hash, the height and the coinbase reward. Running a bitcoin node again, is quite resource intensive - and the disk space alone is 405Gb (standard, not txindex) - I can fully see a situation where some clever software simply listening to the headers could work.
But, running a full node is a necessity if you want to be accurate and sure. Listening to an SPV node increases your chances to be beaten in propagation as you receive the info later than the others. (And you're trusting an entity that can set you up)
as long as you can produce correct nbits for the next block (15 lines of c++), validate the previous header hash (using openssl lib, 5 lines of c++), you can test to see whether the blockheader you received is indeed accurate. not including transactions (since you wouldnt know or be listening for txes) would actually improve your chances of nailing a block.
james