It's part zonal, part man marking. Tallest players doing zonal marking. Someone like Trippier man marking one of their best headers, with the job of getting in their way, not letting them get a free run up.
Whenever something other than old fashioned man marking fails people blame the style of defending. Very rarely will people blame the style of defending when it's old fashioned man marking. Even though many of the best managers in the world choose other ways of defending set plays.
We have the third lowest goals conceded from set pieces in the PL this season. Tied with City, one more than Liverpool, two more than Leicester (I imagine well within statistical variance). Whatever we're doing seems to be working rather well.