How the fuck is he surprised by that? We were the second side to pick it up last season after Chelsea, and played it most of that season!
We used it prior to Chelsea, Poch tried it once or twice the season before Conte came. Most notably against Watford when Troy Denney and his Igalho where looking a effective double team. He has toyed with it for quite some time.
I get the feeling in his heart he wants 4231 but we don't have the mobile/explosive AM's for the role but 3421 suits our squad more and he has become more pragmatic over time.