^ you are pretty much spot on their mate.
Some riders equip their bikes with handguards for that very reason, however I have not seen many over here on the British MX scene some tracks being sand, clay or topsoil even.
Only stony track I raced at was on a farm that had to be graded before a meeting, ie around a foot of soil was added on top of the track due to the ground being flinty,
Still after a couple of hours and a few races this didn't prevent the flints coming through and slashing riders tyres.