For me, the idea of a string having a probability is absurd - I can't parse it at all. Let me clarify this with an example: what is the probability of a chair? Both a chair and a string are objects. Ok, the string is an informational entity, not a physical object. What is the probability of an equation? Neither of these questions make any sense.
Now, if we want it to make sense, we must start to expand the question. What is the probability that a 500-bit string will occur? Still not good enough - out of thin air? In my daily emails? So let's try again: what is the probability of a 500-bit string occuring in the following experiment: "write down 500 bits"? Well, 1 if you do it, 0 if you don't. In NO case is it going to be any other value.
How do you fill the blanks so that "what is the probability of a 500-bit string ..." gives you any other result? Please email me if you found a way.