Are files download times actually unknowable due to the halting problem?
Not at all.
The halting problem says it is impossible to decide whether an arbitrary turing machine with arbitrary input will halt. But there are many, many specific turing machines where we can prove that they halt for every input, or where we can look at the input and decide if the turing machine will halt or not.
And since the people writing the download code surely wrote it in such a way that it is guaranteed to finish for every import, there will be a proof that it is guaranteed to finish.