For me this is pretty clear: See picture below. When the ball is played out towards Benteke it's rolling towards the corner flag, Smalling needs to move in closer and cut off the option for Benteke to bring it over to his right and into the danger area.
Instead of going in closer, smalling positions himself so that Benteke can either go down towards the line for a cross, or shift it over on his right and go inwards towards goal:
The latter is problematic because:
- It opens up space out wide for someone overlapping
- Benteke will move closer to goal, opening for a pass or an attempted shot from just outside the box.
In the end he opts for the easy pass to Townsend and yes it goes via Lindy, but it's difficult to say if De Gea had saved it anyway.
I'm not sure why anyone would describe this as fine defending.
Amusingly enough, Valencia is hardly doing a fine job defending here either. He runs in far past Smalling and just keeps his distance