It has absolutely everything to do with spreads, and the CME and ICE internal order matching spread engine.
Set your trading front-end to see the implied pricing. On ICE the flat price block trades are printed seperately on WebICE. On CME, the Clearport flat price block trades are not printed on the CME Nymex price/volume feed for the futures.
That 400 lot is a spread order getting matched internally at CME. It is real and it counts, but isn't a factor in terms of flat price per se.
Set your front end for implieds. It will open up your eyes. Energy is a spreader's market.