Guessing you were riding the Danebury 150, can't say I've seen anyone riding an brevet / Audax without clipless pedals, they are an almost standard part of bike attire.
There is normally a mix of road an MTB pedals / shoes being used, using MTB pedals likes Shimano Time or Crank Bros gives the rider the advantage of using shoes which can be walked in, where road pedals are harder to walk in.
For making a difference, with clipless pedals, you can pull up on the pedals and do circles far easier than without.