The thumb safety was not put there solely for soldiers on horseback.
Nonetheless, the thumb safety was the request of the US Cavalry...for hasty reholstering when the mounted trooper needed both hands to get control of a horse.
but it doesn’t have a firing pin block. 1911's can discharge if dropped on the muzzle without the block.
So what?
It has to hit dead vertically on a hard surface when dropped from any realistic height...as in less than 10-12 feet. If a fired bullet struck a vertical concrete wall, it would stop and fall to the ground. At most, it would bounce off and land 3-4 feet away. Why would it behave any differently because it was still inside a barrel when it hit?
About the greatest danger would be flying concrete debris at around shoe level. And if it did happen to fire when dropped on the ground, the bullet would go into the ground. No blood. No foul.
The grip safety...also a cavalry requirement...is a more important drop safety. Because of the heavy, straight-line trigger, the gun could conceivably discharge should it hit the ground muzzle, up...which is more likely than muzzle straight down...endangering both horse and rider. Muzzle down discharge wasn't a great concern.
Cocked, unlocked carry sounds scary, but guns don't "go off" in holsters any more than they go off lying on a table. They only discharge when they're being handled...when the trigger is pulled. A 1911 is no different.
Cocked and unlocked in the holster, the grip safety is still blocking the trigger and the half cock is still there should both hammer hooks shear at the same time, which I've never known to happen in the 50+ years I've spent with the pistol.
And, no. The thumb safety won't block the hammer should both hooks fail.