Actually this is a very conservative estimate, which assumes a large chainring. If the chainring were halved the torque would double. Likewise if the rear sprocket were larger. Since builders of freewheels don't know what sprockets will be used they have to allow for the most unfavorable situation, the limiting factor being chain strength.

I'd estimate that a range of 250 - 300 ft#s of torque would still be a conservative estimate ot their capacity.

Consider that the mechanism is the same as used on freewheels which use sprockets of 30t or more, driven by grannies. If you use Doohickie's analysis assuming a 24/30 combination you'd see that even my estimate might be conservative.

Obviously quality of construction is a factor, but the basic design is at the top of the scale for torque capacity in the category of over-running clutches.