They're sampling slope and bias for the line from U[0,1] and U[0,100], the expected values of which will be 0.5 and 50. That's why they get what they get.
They say that randomness is any positively sloped line with any positive bias. So they assume dependence, just a very high variance of it. It's incorrect as they miss the negative half of the parameters - it would then correctly yield zeroes, for the presumed independence.
Is it also plausible that the relationship between true and percieved is wholly independent (as this presumes)?