Decidability of language containing strings of length equal to that of some substring of 1s in $\pi$?
You are confusing knowledge and truth. Something may be true without us knowing precisely why. An object may exist without us being able to construct it.

If you belive in classical logic, then it is true that the maximum length of consequtive 1's in the expansion of $\pi$ is either finite or infinite. In each case, there exists a decision procedure for your language. Therefore a decision procedure exists.

But we do not know which of the cases happens, and so we not not know a particular decision procedure, even though one exists. If you don't like this line of reasoning, then you should ask for a constructive proof of decidability of your language.