The muck from infected animals and the mud it contaminates probably (and I say probably because it’s been 10 years since my last microbiology lecture) has a far higher viral load than the air, so cleaning your boots is probably a reasonable idea. Also, the DEFRA (or whatever they’re called these days) officials and vets will use different overalls at different farms, whilst farmers probably won’t visit each other’s farms. All in all, they’re reasonable precautions to take and cost very little to implement.
All that being said several farms (7 out of 12 infact) in the 2001 outbreak are thought to have been infected by airbourne transmission, the remaining farms were infected by other routes. An outbreak in the Netherlands was spread by milk being illegally discharged into a sewer.