I'm running this play breakdown as a picture instead of a GIF because this play doesn't serve well as a GIF.
The key here is how the Steelers' secondary looks after the play fake. Safety Ryan Clark is still in the deep secondary, but upon seeing TE Jermaine Gresham run an out, and seeing Green go past him deep, the fact he's shadowing Green shows he's in a zone defense.
Also, Taylor's technique off the snap in his coverage of Green is indicative of zone coverage.
This is a well-disguised version of a Tampa 2 coverage scheme.
Outside linebacker James Harrison drops into a hook-to-curl zone after confirming pass, and strong safety Will Allen run-blitzes off the tackle, so Clark has the middle zone. Taylor and cornerback Keenan Lewis are actually the deep zone defenders.
It's been reported by some and written about by fans that Lewis peeled off his coverage to provide Taylor - who in their opinion was burned by Green on the post - with bail-out help. That's incorrect. Taylor is playing off Green's back shoulder, preventing a post-corner or deep out route, because Taylor has coverage of the offensive left half.
It's a smart defense, especially on third-and-short. Clark is tough enough against the run but smart enough to read the play at the snap and get where he needs to be.
The Steelers defend this play with their base defensive personnel, which is an encouraging sign for the future. They feel comfortable enough to attack these kinds of situations - a good passing deep with an outstanding deep threat in their own territory - without an extra defensive back.
It's a brilliant call by defensive coordinator Dick LeBeau, and perhaps maybe a little underplayed by Lewis, who tipped the pass away from Green in the end zone.
Had Lewis been at the appropriate depth, it's possible Dalton would have seen him and not made the throw, but if he did, he would have been in position for the interception.